These tests were written (and copy-pasted) before ClutterActor
had an actual background-color property. As a preparation to
the removal of ClutterRectangle, replace all these rectangles
with plain actors and background colors.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1332
The property is deprecated and the current implementation simply
redirects it to ClutterActor::background-color, so remove it.
Also update the tests to set the background color directly.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1332
It is deprecated in favor of the 'z-position' property, and
the implementation itself redirects to the z-position, so
just drop it and replace all get|set_depth calls to their
z-position counterparts.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1332