This removes all occurrences of creating a custom stage from the tests, making them always use the one from MetaBackend. This will allow for further cleanups. https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1364
Clutter doesn't hold anymore backend implementations, move tests where we have one that we may assign. https://gitlab.gnome.org/GNOME/mutter/merge_requests/672