net/http: do not listen on public network during HTTP/2 test

Avoids Mac firewall box.

Change-Id: I000e421fa9639612d636b6fa4baf905459c5aeb2
Reviewed-on: https://go-review.googlesource.com/16514Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -1070,10 +1070,7 @@ func TestTLSServer(t *testing.T) {
 }
 
 func TestAutomaticHTTP2(t *testing.T) {
-	ln, err := net.Listen("tcp", ":0")
-	if err != nil {
-		t.Fatal(err)
-	}
+	ln := newLocalListener(t)
 	ln.Close() // immediately (not a defer!)
 	var s Server
 	if err := s.Serve(ln); err == nil {