tests: Run a localed mock implementation on tests

As per this we can just warn if its proxy initialization fails.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1233>

src/compositor/plugins/default.c

@@ -396,8 +396,7 @@ init_keymap (MetaDefaultPlugin *self)
                                          &error);
   if (!proxy)
     {
-      g_message ("Failed to acquire org.freedesktop.locale1 proxy: %s, "
-                 "probably running in CI",
+      g_warning ("Failed to acquire org.freedesktop.locale1 proxy: %s",
                  error->message);
       return;
     }