QFINDTESTDATA does not find data relative to test source

With a test in {srcdir}/autotests and a data file in {srcdir}/autotests/data/testfile QFINDTESTDATA("data/testfile") fails to find the file with Qt6 but works fine in Qt 5.15.

The issue boils down to the fact that QT_TESTCASE_BUILDDIR is not defined when building against Qt6.

In Qt5 the following code would have it defined:

find_package(Qt5 REQUIRED COMPONENTS Core Test)add_executable(foo main.cpp)
target_link_libraries(foo PRIVATE Qt::Core Qt::Test)

This is done in Qt5TestConfigExtras.cmake

However this is not the case when building against Qt6. There QT_TESTCASE_BUILDDIR is only defined in QtTestHelpers.cmake which does not seem to be public