Unified comments in unit tests

[sonniki]

Our doc about writing unit tests should mention that we strongly encourage the use of consistent concise comments in unit tests that establish a unified unit test structure:

# Prepare inputs.
...
# Run.
...
# Check.
...

FYI @gpsaggese

[gpsaggese]

Yes indeed. Let's add it to the conventions.

This is another task that (maybe?) an LLM can perform automatically with humans to review it.
My take is to have automatic ways for some of the code maintainers to patch the code already existing and for LLMs to do the first line of defense on a PR (saying what we say all the times!)
Humans should follow the conventions for new code.