CI: Run tests against all renderers that actually open windows
This is a followup to !7472 (merged)
It seems we do not have any tests in CI that actually make sure that all renderers can be used to render to a window. testsuite/gsk
only renders to textures and reftests only use the default renderer.
It might make sense to make different CI runs use different renderers when they run the testsuite.
It might also make sense to write tests specifically to iterate through all the renderers.
It might additionally make sense to ensure we test all backend/renderer combos, not just all renderers.
Not sure which option is best - it might not be fun to analyze rendering differences between renderers in reftests for example.