Test: Introduce order independent testId to replace testNumber

This is a POC. Bikeshed please.
It was the former; adherence to principle on small things so it becomes habit for big things... mostly, I don't see a compelling reason for dropping `testNumber` filtering without notice and it doesn't really cost anything in case someone is using it. But if @jzaefferer is behind its immediate abandonment, I'm not going to stand in the way.