tst_shadersource fails

This blocks the submodule update in https://codereview.qt-project.org/c/qt/qtdeclarative/+/370608 .

Running the test in a parked VM works however.

[qt@10-225-146-210 tests]$ ninja shadersource_check
[1/1] Running /opt/cmake-3.21.1/bin/ctest -V -R "^shadersource$"
UpdateCTestConfiguration from :/home/qt/work/tests/build_dir/DartConfiguration.tcl
UpdateCTestConfiguration from :/home/qt/work/tests/build_dir/DartConfiguration.tcl
Test project /home/qt/work/tests/build_dir
Constructing a list of tests
Done constructing a list of tests
Updating test list for fixtures
Added 0 tests to meet fixture requirements
Checking test dependency graph...
Checking test dependency graph end
test 1
Start 1: shadersource

1: Test command: /opt/cmake-3.21.1/bin/cmake "-P" "/home/qt/work/tests/build_dir/shadersourceWrapperRelWithDebInfo.cmake"
1: Test timeout computed to be: 10000000
1: ********* Start testing of qmltestrunner *********
1: Config: Using QtTest library 6.3.0, Qt 6.3.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 10.2.1 20201112 (Red Hat 10.2.1-8)), rhel 8.4
1: PASS : qmltestrunner::shadersource-dynamic-sourceobject::initTestCase()
1: BPASS : qmltestrunner::shadersource-dynamic-sourceobject::test_endresult()
1: PASS : qmltestrunner::shadersource-dynamic-sourceobject::cleanupTestCase()
1: PASS : qmltestrunner::shadersource-dynamic-shadersource::initTestCase()
1: SKIP : qmltestrunner::shadersource-dynamic-shadersource::test_endresult() grabImage crashes on the xcb platform
1: Loc: [/home/qt/work/qt/qtdeclarative/tests/auto/qmltest/shadersource/tst_DynamicallyCreatedSource.qml(75)]
1: PASS : qmltestrunner::shadersource-dynamic-shadersource::cleanupTestCase()
1: QWARN : qmltestrunner::UnknownTestFunc() QQuickItem: Cannot use same item on different windows at the same time.
1: PASS : qmltestrunner::shadersource-from-other-window::initTestCase()
1: PASS : qmltestrunner::shadersource-from-other-window::test_endresult()
1: QWARN : qmltestrunner::shadersource-from-other-window::test_endresult() Updates can only be scheduled from GUI thread or from QQuickItem::updatePaintNode()
1: QWARN : qmltestrunner::shadersource-from-other-window::test_endresult() Updates can only be scheduled from GUI thread or from QQuickItem::updatePaintNode()
1: QWARN : qmltestrunner::shadersource-from-other-window::test_endresult() Updates can only be scheduled from GUI thread or from QQuickItem::updatePaintNode()
1: PASS : qmltestrunner::shadersource-from-other-window::cleanupTestCase()
1: PASS : qmltestrunner::shadersource-something-to-null::initTestCase()
1: PASS : qmltestrunner::shadersource-something-to-null::test_null()
1: PASS : qmltestrunner::shadersource-something-to-null::cleanupTestCase()
1: PASS : qmltestrunner::shadersource-from-other-window::initTestCase()
1: PASS : qmltestrunner::shadersource-from-other-window::test_endresult()
1: PASS : qmltestrunner::shadersource-from-other-window::cleanupTestCase()
1: QWARN : qmltestrunner::UnknownTestFunc() file:///home/qt/work/qt/qtdeclarative/tests/auto/qmltest/shadersource/tst_SourcedFromOtherWindow.qml:63:9: Unable to assign [undefined] to bool
1: QWARN : qmltestrunner::UnknownTestFunc() QQuickItem: Cannot use same item on different windows at the same time.
1: QWARN : qmltestrunner::UnknownTestFunc() QQuickItem: Cannot use same item on different windows at the same time.
1: Totals: 13 passed, 0 failed, 1 skipped, 1 blacklisted, 369ms
1: ********* Finished testing of qmltestrunner *********
1/1 Test #1: shadersource ..................... Passed 0.72 sec

The following tests passed:
shadersource

100% tests passed, 0 tests failed out of 1

Label Time Summary:
../../../../../qt/qtdeclarative/tests/auto/qmltest/shadersource/shadersource = 0.72 sec*proc (1 test)

Total Test time (real) = 0.72 sec
[qt@10-225-146-210 tests]$ echo $?
0
[qt@10-225-146-210 tests]$ ctest
Test project /home/qt/work/tests
Start 1: shadersource
1/1 Test #1: shadersource ..................... Passed 0.74 sec

100% tests passed, 0 tests failed out of 1

Label Time Summary:
../../../../../qt/qtdeclarative/tests/auto/qmltest/shadersource/shadersource = 0.74 sec*proc (1 test)

Total Test time (real) = 0.74 sec

Note further that even in the CI, the test itself succeeds according to its output, but we then still notice a failure from somewhere