tstest/integration/testcontrol: fix AddRawMapResponse race condition

Only send a stored raw map message in reply to a streaming map response.
Otherwise a non-streaming map response might pick it up first, and
potentially drop it. This guarantees that a map response sent via
AddRawMapResponse will be picked up by the main map response loop in the
client.

Fixes #15362

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -839,15 +839,17 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 
 	w.WriteHeader(200)
 	for {
+		// Only send raw map responses to the streaming poll, to avoid a
+		// non-streaming map request beating the streaming poll in a race and
+		// potentially dropping the map response.
+		if streaming {
 			if resBytes, ok := s.takeRawMapMessage(req.NodeKey); ok {
 				if err := s.sendMapMsg(w, compress, resBytes); err != nil {
 					s.logf("sendMapMsg of raw message: %v", err)
 					return
 				}
-			if streaming {
 				continue
 			}
-			return
 		}
 
 		if s.canGenerateAutomaticMapResponseFor(req.NodeKey) {