tests/clutter/utils: Add helper to flush input thread

This helps making sure input events we inject have managed to reach the
main thread.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2727>

src/tests/clutter-test-utils.h

@@ -113,6 +113,9 @@ void            clutter_test_add_data_full      (const char     *test_path,
                                                  gpointer        test_data,
                                                  GDestroyNotify  test_notify);
 
+CLUTTER_EXPORT
+void            clutter_test_flush_input        (void);
+
 CLUTTER_EXPORT
 ClutterActor *  clutter_test_get_stage          (void);