tstest/integration/testcontrol: fix data race

Noticed in earlier GitHub actions failure.

Fixes #11994

Change-Id: Iba8d753caaa3dacbe2da9171d96c5f99b12e62d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -13,6 +13,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"maps"
 	"math/rand"
 	"net/http"
 	"net/http/httptest"
@@ -914,7 +915,12 @@ func (s *Server) MapResponse(req *tailcfg.MapRequest) (res *tailcfg.MapResponse,
 		// node key rotated away (once test server supports that)
 		return nil, nil
 	}
-	node.CapMap = s.nodeCapMaps[nk]
+
+	s.mu.Lock()
+	nodeCapMap := maps.Clone(s.nodeCapMaps[nk])
+	s.mu.Unlock()
+
+	node.CapMap = nodeCapMap
 	node.Capabilities = append(node.Capabilities, tailcfg.NodeAttrDisableUPnP)
 
 	user, _ := s.getUser(nk)