The first monitor in stacking tests is the primary monitor but that
doesn't have to stay this way forever. Instead of special casing the
name "primary" to refer to whatever monitor happens to be the primary
monitor, we add an `assert_primary_monitor` command to verify that the
monitor that should be the primary monitor actually is.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2748>
This test ensures that windows that were resized such that they extend
beyond the screen will be moved to be fully on the screen (if possible).
This has been working on X11 since forever, but on Wayland only since
the last commit.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2103>