net: disable another external network test

I don't know enough about multicast.
Should this be disabled on all systems, not just Windows?

R=golang-dev
CC=golang-dev
https://golang.org/cl/5754060

src/pkg/net/multicast_test.go

@@ -90,6 +90,11 @@ func TestSimpleMulticastListener(t *testing.T) {
 	case "plan9":
 		t.Logf("skipping test on %q", runtime.GOOS)
 		return
+	case "windows":
+		if testing.Short() || !*testExternal {
+			t.Logf("skipping test on windows to avoid firewall")
+			return
+		}
 	}
 
 	for _, tt := range multicastListenerTests {