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/monitor-unit-tests.c

@@ -343,6 +343,8 @@ create_monitor_test_clients (void)
 {
   GError *error = NULL;
 
+  test_wait_for_x11_display ();
+
   meta_x11_display_set_alarm_filter (meta_get_display ()->x11_display,
                                      monitor_tests_alarm_filter, NULL);