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/cmd/mattermost/commands/server_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 const (
-	unitTestListeningPort = ":0"
+	unitTestListeningPort = "localhost:0"
 )
 
 //nolint:golint,unused