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-utils.c

@@ -533,3 +533,17 @@ test_get_plugin_name (void)
   else
     return "libdefault";
 }
+
+void
+test_wait_for_x11_display (void)
+{
+  MetaDisplay *display;
+
+  display = meta_get_display ();
+  g_assert_nonnull (display);
+
+  while (!display->x11_display)
+    g_main_context_iteration (NULL, TRUE);
+
+  g_assert_nonnull (display->x11_display);
+}