main: Make it possible to set properties when overriding configuration

This makes it possible to pass custom properties to backends when
constructing tests. This will be used to create "headless" native
backend instances for testing the headless native backend.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>

src/tests/stage-view-tests.c

@@ -1192,7 +1192,8 @@ main (int argc, char *argv[])
   meta_plugin_manager_load (test_get_plugin_name ());
 
   meta_override_compositor_configuration (META_COMPOSITOR_TYPE_WAYLAND,
-                                          META_TYPE_BACKEND_TEST);
+                                          META_TYPE_BACKEND_TEST,
+                                          NULL);
 
   meta_init ();
   meta_register_with_session ();