How do I test my websocket server in go?

type Websocket struct {
	upgrader *websocket.Upgrader

	CmdProxy proxy.CmdProxy

	connections map[uint64]map[*websocket.Conn]bool

	init sync.Once
	lock sync.Mutex
}

func (w *Websocket) Upgrade(thingId uint64, wr http.ResponseWriter, r *http.Request) error {
	w.init.Do(func() {
		w.connections = make(map[uint64]map[*websocket.Conn]bool)
		w.upgrader = &websocket.Upgrader{
			CheckOrigin: func(r *http.Request) bool { return true },
			Error:       func(w http.ResponseWriter, r *http.Request, status int, reason error) {},
		}
	})

	conn, err := w.upgrader.Upgrade(wr, r, map[string][]string{
		"Sec-Websocket-Protocol": {r.Context().Value(services.TokenKey).(string)},
	})
	if err != nil {
		return err
	}

	w.lock.Lock()
	connections, exists := w.connections[thingId]
	if !exists {
		connections = make(map[*websocket.Conn]bool)
	}
	connections[conn] = true
	w.connections[thingId] = connections

	w.lock.Unlock()
	// start new handling
	w.handle(thingId, conn)
	return nil
}

Hello everyone, This is my WebSocket Upgrade function. I want to test this golang code. since I am new to the testing world i am raising this issue here especially about the response write which is an interface and i don't know how to mock it.