tests: Add missing stacking tests

List all .metatest files that were added only to autotools while they are
missing since the meson port.

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

src/tests/meson.build

@@ -104,10 +104,13 @@ headless_start_test = executable('mutter-headless-start-test',
 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',
 ])
 
 test('mutter/stacking', test_runner,