chain: resolve data race in sequential start stop test

Resolve the data race in the sequential start stop unit test
by ensuring that each goroutine launched in the Start method
is properly accounted for in the client's waitgroup.

chain/neutrino.go

@@ -99,18 +99,26 @@ func (s *NeutrinoClient) Start() error {
 	s.clientMtx.Lock()
 	defer s.clientMtx.Unlock()
 	if !s.started {
+		// Reset the client state.
 		s.enqueueNotification = make(chan interface{})
 		s.dequeueNotification = make(chan interface{})
 		s.currentBlock = make(chan *waddrmgr.BlockStamp)
 		s.quit = make(chan struct{})
 		s.started = true
+
+		// Go place a ClientConnected notification onto the queue.
 		s.wg.Add(1)
 		go func() {
+			defer s.wg.Done()
+
 			select {
 			case s.enqueueNotification <- ClientConnected{}:
 			case <-s.quit:
 			}
 		}()
+
+		// Go launch the notification handler.
+		s.wg.Add(1)
 		go s.notificationHandler()
 	}
 	return nil