tests: Use suites for test cases

They allows to filter tests better and so we can just launch tests with:
  meson test --suite [core|cogl|clutter] [single-test-name]

https://gitlab.gnome.org/GNOME/mutter/merge_requests/442

clutter/tests/conform/meson.build

@@ -74,7 +74,8 @@ foreach test : clutter_conform_tests
     install: false,
   )
 
-  test('clutter/conform/@0@'.format(test), test_executable,
+  test(test, test_executable,
+    suite: ['clutter', 'clutter/conform'],
     env: test_env
   )
 endforeach