Github Workflow: trigger a retest

[dominykas]

At the moment, when a wiby test label is added, it will kick off the workflow, which will run wiby test if there is no status check for wiby; if there is a pending status check, it will instead run a wiby result and report the status.

There currently is no way to clean up the status check, therefore there is no way to re-start the wiby test if for some reason the tests got stuck and the results are never moving past "pending".

Ideas:

• add support for a wiby clean test label, which would clear the status check, run a wiby clean and only then kick off a wiby test
  • gotcha: as wiby clean closes pending PRs / deletes pending branches, it might destroy any user changes in these branches
• add support for a wiby retest label, which would clear the status check and re-run wiby test
  • gotcha: we probably then also need an explicit wiby clean label?
• regardless of anything, we should probably remove all the wiby * trigger labels at the end of any workflow run