This launches Xvfb, using xvfb-run, and inside tests the following:
1. Launching 'mutter --x11' works
2. Launching a couple of X11 clients works (doesn't crash or result in
warnings)
3. Launching 'mutter --x11 --replace' works
4. Terminating works
It does this using a simple shell script.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2434>
