net/netcheck: add check for captive portal (#5593)

This doesn't change any behaviour for now, other than maybe running a
full netcheck more often. The intent is to start gathering data on
captive portals, and additionally, seeing this in the 'tailscale
netcheck' command should provide a bit of additional information to
users.

Updates #1634

Change-Id: I6ba08f9c584dc0200619fa97f9fde1a319f25c76
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

tstest/integration/testcontrol/testcontrol.go

@@ -200,6 +200,9 @@ func (s *Server) logf(format string, a ...any) {
 func (s *Server) initMux() {
 	s.mux = http.NewServeMux()
 	s.mux.HandleFunc("/", s.serveUnhandled)
+	s.mux.HandleFunc("/generate_204", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	})
 	s.mux.HandleFunc("/key", s.serveKey)
 	s.mux.HandleFunc("/machine/", s.serveMachine)
 }