Avoid macOS prompt to allow incoming connections in liveshare tests

Listening on the localhost interface disallows connections from the
outside anyway, so the OS firewall doesn't have to prompt whether the
user wants to allow incoming connections to the Go process.

pkg/liveshare/port_forwarder_test.go

@@ -46,7 +46,7 @@ func TestPortForwarderStart(t *testing.T) {
 	}
 	defer testServer.Close()
 
-	listen, err := net.Listen("tcp", ":8000")
+	listen, err := net.Listen("tcp", "127.0.0.1:8000")
 	if err != nil {
 		t.Fatal(err)
 	}