This will later be used to allow skipping tests. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2151>
This will require some symbol exporting, but the benefit is that have better control of what external test cases can do in terms of creating more testing specific contexts. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>