The D-Bus runner used by tests, including installed tests, is made to be
reusable from GNOME Shell. To do this, install it and the templates in
the pkgdatadir (e.g. /usr/share/mutter-APIVERSION/tests/), generate a
custom runner for the installed tests that uses the installed script and
templates, and change the non-installed original runner to use the
non-installed templates.
The end goal is to reuse the D-Bus session runner and templates used for
mutter when test running GNOME Shell.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1354>
