tests: Add single stacking tests with suite

Don't launch the stacking tests in one single shot, to allow better debugging
and being able to launch just one single test using meson test.

Those tests can now be all launched with:
  meson test --suite stacking [single-test-name]

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

src/tests/meson.build

@@ -101,26 +101,29 @@ headless_start_test = executable('mutter-headless-start-test',
   install_dir: mutter_installed_tests_libexecdir,
 )
 
-stacking_tests = files([
-  'stacking/basic-x11.metatest',
-  'stacking/basic-wayland.metatest',
-  'stacking/client-side-decorated.metatest',
-  'stacking/closed-transient.metatest',
-  'stacking/minimized.metatest',
-  'stacking/mixed-windows.metatest',
-  'stacking/set-parent.metatest',
-  'stacking/override-redirect.metatest',
-  'stacking/set-parent-exported.metatest',
-])
+stacking_tests = [
+  'basic-x11',
+  'basic-wayland',
+  'client-side-decorated',
+  'closed-transient',
+  'minimized',
+  'mixed-windows',
+  'set-parent',
+  'override-redirect',
+  'set-parent-exported',
+]
 
-test('mutter/stacking', test_runner,
+foreach stacking_test: stacking_tests
+  test(stacking_test, test_runner,
+    suite: ['core', 'mutter/stacking'],
     env: test_env,
     args: [
-    stacking_tests,
+      files(join_paths('stacking', stacking_test + '.metatest')),
     ],
     is_parallel: false,
     timeout: 60,
-)
+  )
+endforeach
 
 test('mutter/unit', unit_tests,
   env: test_env,