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.c

@@ -66,6 +66,12 @@ meta_context_set_plugin_name (MetaContext *context,
   priv->plugin_name = g_strdup (plugin_name);
 }
 
+void
+meta_context_notify_ready (MetaContext *context)
+{
+  META_CONTEXT_GET_CLASS (context)->notify_ready (context);
+}
+
 static MetaCompositorType
 meta_context_get_compositor_type (MetaContext *context)
 {