treesorter tests: Add a comment

This looks like a leak, but isn't one.
Add a comment to that effect.

testsuite/gtk/treesorter.c

@@ -69,6 +69,7 @@ model_to_string (GListModel *model)
       if (i > 0)
         g_string_append (string, " ");
       g_string_append_printf (string, "%u", get_with_parents (object));
+      /* no unref since get_with_parents consumes the ref */
     }
 
   return g_string_free (string, FALSE);