kmime fails two tests with Qt 5.15.0-beta2. I don't remember them failing before and they don't fail on another machine with 5.14.2:

6: Test command: /var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/build/bin/headertest
6: Environment variables: 
6:  LC_TIME=C
6: Test timeout computed to be: 10000000
6: ********* Start testing of HeaderTest *********
6: Config: Using QtTest library 5.15.0, Qt 5.15.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 10.0.1 20200322 (experimental))
6: PASS   : HeaderTest::initTestCase()
6: XFAIL  : HeaderTest::testIdentHeader() Parsing strips square brackets.
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(86)]
6: PASS   : HeaderTest::testIdentHeader()
6: FAIL!  : HeaderTest::testAddressListHeader() Compared values are not the same
6:    Actual   (h->as7BitString(false))                                          : "Ingo =?ISO-8859-15?Q?Kl=F6cker?= <kloecker@kde.org>"
6:    Expected (QByteArray("Ingo =?ISO-8859-1?Q?Kl=F6cker?= <kloecker@kde.org>")): "Ingo =?ISO-8859-1?Q?Kl=F6cker?= <kloecker@kde.org>"
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(161)]
6: PASS   : HeaderTest::testMailboxListHeader()
6: FAIL!  : HeaderTest::testSingleMailboxHeader() Compared strings are not the same
6:    Actual   (h->mailboxes().first().name().toLatin1().data()): Andre  Mr.  Tall  Woebbeking
6:    Expected ("Andre \"Mr. Tall\" Woebbeking")                : Andre "Mr. Tall" Woebbeking
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(389)]
6: PASS   : HeaderTest::testMailCopiesToHeader()
6: PASS   : HeaderTest::testParametrizedHeader()
6: PASS   : HeaderTest::testContentDispositionHeader()
6: FAIL!  : HeaderTest::testContentTypeHeader() Compared values are not the same
6:    Actual   (h->name()):         ""
6:    Expected (QString::fromUtf8("lorém ipsum.txt")): "lor\u00E9m ipsum.txt"
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(602)]
6: PASS   : HeaderTest::testTokenHeader()
6: PASS   : HeaderTest::testContentTransferEncoding()
6: PASS   : HeaderTest::testPhraseListHeader()
6: PASS   : HeaderTest::testDotAtomHeader()
6: PASS   : HeaderTest::testDateHeader()
6: PASS   : HeaderTest::testLinesHeader()
6: PASS   : HeaderTest::testNewsgroupsHeader()
6: PASS   : HeaderTest::testControlHeader()
6: PASS   : HeaderTest::testReturnPath()
6: PASS   : HeaderTest::testInvalidButOkQEncoding()
6: PASS   : HeaderTest::testInvalidQEncoding()
6: PASS   : HeaderTest::testInvalidQEncoding()
6: PASS   : HeaderTest::testInvalidQEncoding()
6: PASS   : HeaderTest::testBug271192(Plain)
6: FAIL!  : HeaderTest::testBug271192(Firstname_1) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: FAIL!  : HeaderTest::testBug271192(Firstname_2) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: FAIL!  : HeaderTest::testBug271192(Lastname) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: FAIL!  : HeaderTest::testBug271192(Firstname_Lastname) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: PASS   : HeaderTest::testBug271192(Quotemarks)
6: FAIL!  : HeaderTest::testBug271192(Quotemarks_nonascii) Compared values are not the same
6:    Actual   (h->displayNames().first().toUtf8())              : "J\xC3\xB6hn \\\"R\xC3\xB6""cky\\\" D\xC3\xB6""e"
6:    Expected (displayName.remove(QLatin1String("\\")).toUtf8()): "J\xC3\xB6hn \"R\xC3\xB6""cky\" D\xC3\xB6""e"
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1045)]
6: PASS   : HeaderTest::testBug271192(quote_Plain)
6: FAIL!  : HeaderTest::testBug271192(quote_Firstname_1) Compared values are not the same
6:    Actual   (h->displayNames().size()): 0
6:    Expected (1)                       : 1
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1044)]
6: FAIL!  : HeaderTest::testBug271192(quote_Firstname_2) Compared values are not the same
6:    Actual   (h->displayNames().size()): 0
6:    Expected (1)                       : 1
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1044)]
6: FAIL!  : HeaderTest::testBug271192(quote_Lastname) Compared values are not the same
6:    Actual   (h->displayNames().size()): 0
6:    Expected (1)                       : 1
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1044)]
6: FAIL!  : HeaderTest::testBug271192(quote_Firstname_Lastname) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: FAIL!  : HeaderTest::testBug271192(quote_LastName_comma_Firstname) Compared values are not the same
6:    Actual   (h2->displayNames().size()): 1
6:    Expected (2)                        : 2
6:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/headertest.cpp(1051)]
6: PASS   : HeaderTest::testMissingQuotes()
6: PASS   : HeaderTest::noAbstractHeaders()
6: PASS   : HeaderTest::cleanupTestCase()
6: Totals: 25 passed, 13 failed, 0 skipped, 0 blacklisted, 11ms
6: ********* Finished testing of HeaderTest *********
 6/18 Test  #6: kmime-headertest .................***Failed    0.02 sec

8: Test command: /var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/build/bin/messagetest
8: Environment variables: 
8:  LC_TIME=C
8: Test timeout computed to be: 10000000
8: ********* Start testing of MessageTest *********
8: Config: Using QtTest library 5.15.0, Qt 5.15.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 10.0.1 20200322 (experimental))
8: PASS   : MessageTest::initTestCase()
8: PASS   : MessageTest::testMainBodyPart()
8: PASS   : MessageTest::testBrunosMultiAssembleBug()
8: PASS   : MessageTest::testWillsAndTillsCrash()
8: PASS   : MessageTest::testDavidsParseCrash()
8: PASS   : MessageTest::testHeaderFieldWithoutSpace()
8: PASS   : MessageTest::testWronglyFoldedHeaders()
8: PASS   : MessageTest::missingHeadersTest()
8: PASS   : MessageTest::testBug219749()
8: PASS   : MessageTest::testBidiSpoofing()
8: PASS   : MessageTest::testUtf16()
8: PASS   : MessageTest::testDecodedText()
8: PASS   : MessageTest::testInlineImages()
8: PASS   : MessageTest::testIssue3908()
8: PASS   : MessageTest::testIssue3914()
8: PASS   : MessageTest::testBug223509()
8: PASS   : MessageTest::testEncapsulatedMessages()
8: FAIL!  : MessageTest::testOutlookAttachmentNaming() Compared values are not the same
8:    Actual   (cd->filename()):              ""
8:    Expected (QString::fromUtf8("å.diff")): "\u00E5.diff"
8:    Loc: [/var/tmp/paludis/build/kde-frameworks-kmime-20.03.90/work/kmime-20.03.90/autotests/messagetest.cpp(584)]
8: PASS   : MessageTest::testEncryptedMails()
8: PASS   : MessageTest::testReturnSameMail()
8: PASS   : MessageTest::testEmptySubject()
8: PASS   : MessageTest::cleanupTestCase()
8: Totals: 21 passed, 1 failed, 0 skipped, 0 blacklisted, 7ms
8: ********* Finished testing of MessageTest *********
 8/18 Test  #8: kmime-messagetest ................***Failed    0.03 sec

SOFTWARE/OS VERSIONS
kmime Version: 20.03.90
KDE Frameworks Version: 5.68.0
Qt Version: 5.15.0-beta2