tests/monitor-transform: Also test invert()

Commit e06daa58c3 changed the tested values to use corresponding valid
enum values instead of negative ones. Unfortunately that made one value
become a duplicate of an existing one and also in part defeated the original
intention of checking the implementation of
`meta_output_crtc_to_logical_transform`.

Use `meta_monitor_transform_invert` to fix both shortcomings.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1242

src/backends/meta-monitor-transform.h

@@ -52,6 +52,7 @@ meta_monitor_transform_is_flipped (MetaMonitorTransform transform)
   return (transform >= META_MONITOR_TRANSFORM_FLIPPED);
 }
 
+META_EXPORT_TEST
 MetaMonitorTransform meta_monitor_transform_invert (MetaMonitorTransform transform);
 
 META_EXPORT_TEST