Meta: show which tests are currently running

[Janiczek]

Sometimes there's a test that takes long / the tested code hits an infinite loop etc., and it's hard to figure out which test is the culprit. If we showed currently running tests in the CLI, it would be easier to find. (Passing tests would hide after finishing, failing tests would probably stay.)

Inspiration:
https://twitter.com/janiczek/status/1741885233396412765?t=GtCzN6GEIKHUn3C-SJcHEg&s=19

Why meta: this would be the job for the runners, not for this library (IMO)