This is in preparation of moving Cogl tests into src/tests, so they can
use the real backend, instead of the franken-backend it some how still
manages to use some how.
This makes them no longer installed. Most mutter tests are yet to be
installed, so leave that for later, since bigger changes are needed for
that.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2555>
