tests: Ensure MetaX11Display is initialized before running tests

It might not be available right on initialization time if X11 is started
asynchronously. As this is a requirement for our tests, ensure it is there
before proceeding with the test.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/944

src/tests/test-runner.c

@@ -66,6 +66,8 @@ test_case_new (void)
 {
   TestCase *test = g_new0 (TestCase, 1);
 
+  test_wait_for_x11_display ();
+
   meta_x11_display_set_alarm_filter (meta_get_display ()->x11_display,
                                      test_case_alarm_filter, test);