tests/test-runner: Add 'sleep suspend_timeout' command

It's a tweak to the 'sleep' command to allow well known sleep lengths.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3213>

src/core/window.c

@@ -7999,3 +7999,9 @@ meta_window_calculate_bounds (MetaWindow *window,
       return FALSE;
     }
 }
+
+int
+meta_get_window_suspend_timeout_s (void)
+{
+  return SUSPEND_HIDDEN_TIMEOUT_S;
+}