tests: Extract D-Bus runner as reusable python module

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>

src/tests/mutter-installed-dbus-session.py.in

@@ -0,0 +1,11 @@
+#!/usr/bin/env python3
+
+import sys
+
+sys.path.insert(1, '@tests_datadir@')
+
+from mutter_dbusrunner import MutterDBusRunner, meta_run
+
+if __name__ == '__main__':
+    result = meta_run(MutterDBusRunner)
+    sys.exit(result)