Enforce offscreen rendering for all tests

[uklotzde]

Enforce offscreen rendering for all tests except benchmark tests.

Background: In the RPM build ctest is invoked through a macro %ctest3 that does not to allow setting initial environment variables. This change finally allows to enable and run the tests in the Fedora build system.

[Holzhaus]

Thanks, this would be nice, but last time I tried the offscreen renderer didn't work on Windows. Maybe because it's not included in the static build? Let's wait for CI.

[uklotzde]

Thanks, this would be nice, but last time I tried the offscreen renderer didn't work on Windows. Maybe because it's not included in the static build? Let's wait for CI.

The tests on Windows stalled with no output. Reverted and pushed a revised version.

[uklotzde]

I am curious to see if all tests will pass on ARM and PowerPC architectures.

[Holzhaus]

Can you make a follow up that removes the QT_QPA_PLATFORM_PLUGIN_PATH from the GitHub Actions build workflow? I think with these changes it's not needed anymore.

[uklotzde]

Can you make a follow up that removes the QT_QPA_PLATFORM_PLUGIN_PATH from the GitHub Actions build workflow? I think with these changes it's not needed anymore.

Ideas how to achieve this in CMakeLists.txt for the mixxx-benchmark target? This would allow complete removal.