We're moving towards not supporting stand-alone application style
clutter stages, meaning the stage tests use will be reused instead of
recreated. To make this feasable, tests must clean up after themself.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1364
And add the necessary glue so those initialize a X11 clutter backend.
This should get Clutter tests that are dependent on windowing to work
again, thus they were enabled back again.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/672