tests/clutter/conform: Don't run tests in parallel

Might end up failing to acquire D-Bus names, resulting in warnings.
Avoid that by not running the tests in parallel.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1289

src/tests/clutter/conform/meson.build

@@ -73,6 +73,7 @@ foreach test : clutter_conform_tests
 
   test(test, test_executable,
     suite: ['clutter', 'clutter/conform'],
-    env: test_env
+    env: test_env,
+    is_parallel: false,
   )
 endforeach