context: Add 'notify_ready()' vfunc and method

This intends to replace the call to `meta_register_with_session()` that
deals with X11 session management, and is called when the user is
"ready". In thet test context, doing that makes no sense, so make it a
no-op.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/meta-context-private.h

@@ -41,6 +41,8 @@ struct _MetaContextClass
 
   MetaBackend * (* create_backend) (MetaContext  *context,
                                     GError      **error);
+
+  void (* notify_ready) (MetaContext *context);
 };
 
 #endif /* META_CONTEXT_PRIVATE_H */