bind to localhost:0 for tests (#22839)

* bind to localhost:0 for tests

Explicitly bind to `localhost:0` instead of just `:0` to avoid binding to all available interfaces and in turn avoid triggering firewall warnings on MacOS.

* fix Playbooks to use dynamic port

* TestMetrics: handle ipv4 localhost too

* TestMetrics: linting

server/channels/app/app_test.go

@@ -22,7 +22,7 @@ func TestAppRace(t *testing.T) {
 	for i := 0; i < 10; i++ {
 		a, err := New()
 		require.NoError(t, err)
-		a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = ":0" })
+		a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = "localhost:0" })
 		serverErr := a.StartServer()
 		require.NoError(t, serverErr)
 		a.Srv().Shutdown()