Soon we'll expose it via a libmutter-test library. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
Otherwise it'll conflict with other files with identical filenames, if one would add src/tests/ to the include path, which will happen in a later commit. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>