[MM-34179] app: move cluster registeration to server creation (#17306)

* app: move cluster registeration to server creation

* initserver via fakeapp

* reflect review comments

app/admin.go

@@ -235,9 +235,9 @@ func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 	return nil
 }
 
-// ServerBusyStateChanged is called when a CLUSTER_EVENT_BUSY_STATE_CHANGED is received.
-func (a *App) ServerBusyStateChanged(sbs *model.ServerBusyState) {
-	a.Srv().Busy.ClusterEventChanged(sbs)
+// serverBusyStateChanged is called when a CLUSTER_EVENT_BUSY_STATE_CHANGED is received.
+func (s *Server) serverBusyStateChanged(sbs *model.ServerBusyState) {
+	s.Busy.ClusterEventChanged(sbs)
 	if sbs.Busy {
 		mlog.Warn("server busy state activitated via cluster event - non-critical services disabled", mlog.Int64("expires_sec", sbs.Expires))
 	} else {