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

src/tests/meson.build

@@ -125,13 +125,15 @@ foreach stacking_test: stacking_tests
   )
 endforeach
 
-test('mutter/unit', unit_tests,
+test('normal', unit_tests,
+  suite: ['core', 'mutter/unit'],
   env: test_env,
   is_parallel: false,
   timeout: 60,
 )
 
-test('mutter/unit/headless-start', headless_start_test,
+test('headless-start', headless_start_test,
+  suite: ['core', 'mutter/unit'],
   env: test_env,
   is_parallel: false,
   timeout: 60,