log-file as third output channel during tests

[cre4ture]

draft 1 for #6284, alternative for #6314

The main idea here is that the logs can be part of the stable code-base. But this means, that there need to be a mechanism to activate then on demand.

This PR gives an example how to achieve this.

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[sylvestre]

i think it complexifies the code a bit too much for my taste