tailcfg, control/controlclient: start moving MapResponse.DefaultAutoUpdate to a nodeattr

And fix up the TestAutoUpdateDefaults integration tests as they
weren't testing reality: the DefaultAutoUpdate is supposed to only be
relevant on the first MapResponse in the stream, but the tests weren't
testing that. They were instead injecting a 2nd+ MapResponse.

This changes the test control server to add a hook to modify the first
map response, and then makes the test control when the node goes up
and down to make new map responses.

Also, the test now runs on macOS where the auto-update feature being
disabled would've previously t.Skipped the whole test.

Updates #11502

Change-Id: If2319bd1f71e108b57d79fe500b2acedbc76e1a6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -79,6 +79,10 @@ type Server struct {
 	ExplicitBaseURL string           // e.g. "http://127.0.0.1:1234" with no trailing URL
 	HTTPTestServer  *httptest.Server // if non-nil, used to get BaseURL
 
+	// ModifyFirstMapResponse, if non-nil, is called exactly once per
+	// MapResponse stream to modify the first MapResponse sent in response to it.
+	ModifyFirstMapResponse func(*tailcfg.MapResponse, *tailcfg.MapRequest)
+
 	initMuxOnce sync.Once
 	mux         *http.ServeMux
 
@@ -993,6 +997,7 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 	// register an updatesCh to get updates.
 	streaming := req.Stream && !req.ReadOnly
 	compress := req.Compress != ""
+	first := true
 
 	w.WriteHeader(200)
 	for {
@@ -1025,6 +1030,10 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 			if allExpired {
 				res.Node.KeyExpiry = time.Now().Add(-1 * time.Minute)
 			}
+			if f := s.ModifyFirstMapResponse; first && f != nil {
+				first = false
+				f(res, req)
+			}
 			// TODO: add minner if/when needed
 			resBytes, err := json.Marshal(res)
 			if err != nil {