test_QT_TESTCASE_BUILDDIR fails when built with CMake 3.21

agent:2021/07/19 10:42:24 build.go:391: 237: Running test command: "/Users/qt/work/qt/qtbase_standalone_tests/tests/auto/cmake/in_source/test_QT_TESTCASE_BUILDDIR/build/test_qt_testcase_builddir"
agent:2021/07/19 10:42:24 build.go:391: 237: Test command failed: /Users/qt/work/qt/qtbase_standalone_tests/tests/auto/cmake/in_source/test_QT_TESTCASE_BUILDDIR/build/test_qt_testcase_builddir
agent:2021/07/19 10:42:24 build.go:391: 237: ********* Start testing of TestClass *********
agent:2021/07/19 10:42:24 build.go:391: 237: Config: Using QtTest library 6.2.0, Qt 6.2.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by Clang 12.0.0 (clang-1200.0.32.29) (Apple)), macos 11.0
agent:2021/07/19 10:42:24 build.go:391: 237: PASS   : TestClass::initTestCase()
agent:2021/07/19 10:42:24 build.go:391: 237: WARNING: TestClass::doTest() testdata level2/testdata_build.txt could not be located!
agent:2021/07/19 10:42:24 build.go:391: 237:    Loc: [/Users/qt/work/qt/qtbase_standalone_tests/tests/auto/cmake/in_source/test_QT_TESTCASE_BUILDDIR/main.cpp(50)]
agent:2021/07/19 10:42:24 build.go:391: 237: QWARN  : TestClass::doTest() QFSFileEngine::open: No file name specified
agent:2021/07/19 10:42:24 build.go:391: 237: FAIL!  : TestClass::doTest() 'fbuild.open(QFile::ReadOnly)' returned FALSE. ()
agent:2021/07/19 10:42:24 build.go:391: 237:    Loc: [/Users/qt/work/qt/qtbase_standalone_tests/tests/auto/cmake/in_source/test_QT_TESTCASE_BUILDDIR/main.cpp(51)]
agent:2021/07/19 10:42:24 build.go:391: 237: PASS   : TestClass::cleanupTestCase()
agent:2021/07/19 10:42:24 build.go:391: 237: Totals: 2 passed, 1 failed, 0 skipped, 0 blacklisted, 1ms
agent:2021/07/19 10:42:24 build.go:391: 237: ********* Finished testing of TestClass *********

# worked
-c
../main.cpp

# doesn't work
-c
/Users/qt/work/qt/st_cmake/tests/auto/cmake/in_source/test_QT_TESTCASE_BUILDDIR/main.cpp