feature/taildrop: add integration test

Taildrop has never had an end-to-end test since it was introduced.

This adds a basic one.

It caught two recent refactoring bugs & one from 2022 (0f7da5c7dc).

This is prep for moving the rest of Taildrop out of LocalBackend, so
we can do more refactorings with some confidence.

Updates #15812

Change-Id: I6182e49c5641238af0bfdd9fea1ef0420c112738
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -573,7 +573,7 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 		if ms, ok := sys.MagicSock.GetOK(); ok {
 			debugMux.HandleFunc("/debug/magicsock", ms.ServeHTTPDebug)
 		}
-		go runDebugServer(debugMux, args.debug)
+		go runDebugServer(logf, debugMux, args.debug)
 	}
 
 	ns, err := newNetstack(logf, sys)
@@ -819,12 +819,20 @@ func servePrometheusMetrics(w http.ResponseWriter, r *http.Request) {
 	clientmetric.WritePrometheusExpositionFormat(w)
 }
 
-func runDebugServer(mux *http.ServeMux, addr string) {
+func runDebugServer(logf logger.Logf, mux *http.ServeMux, addr string) {
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		log.Fatalf("debug server: %v", err)
+	}
+	if strings.HasSuffix(addr, ":0") {
+		// Log kernel-selected port number so integration tests
+		// can find it portably.
+		logf("DEBUG-ADDR=%v", ln.Addr())
+	}
 	srv := &http.Server{
-		Addr:    addr,
 		Handler: mux,
 	}
-	if err := srv.ListenAndServe(); err != nil {
+	if err := srv.Serve(ln); err != nil {
 		log.Fatal(err)
 	}
 }