CI: Use mutter for the x11-full testsuite run
This spawns a mutter WM for the xvfb instance and wraps it in its own dbus session. Also use a larger screen size (1080p) - mostly so we can check the testuite runs with differently sized screens, but alspo because that's today's default screen size. This exposes the issue from #4136 in the testsuite.
wip/otte/x11-testsuite in 39 minutes and 27 seconds (queued for 1 second)13 jobs for