output: Tie the output to the monitor it's part of

Outputs correspond to active connectors, that we translate into
monitors. Make this association more real by adding a pointer back to
the monitor from the output.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1488>

src/tests/monitor-test-utils.c

@@ -289,6 +289,8 @@ check_logical_monitor (MetaMonitorManager             *monitor_manager,
           MetaOutput *output = l_output->data;
           MetaCrtc *crtc;
 
+          g_assert (meta_output_get_monitor (output) == monitor);
+
           if (meta_output_is_primary (output))
             {
               g_assert_null (primary_output);