Add Util functions to make tests with logging of generated test values

[shym]

This has been useful quite a few times for us, for instance to investigate deadlocks or livelocks, and build standalone reproducers. This might be more generally useful.
This is still a draft, with the following points to fix:

• documentation for the new functions
• possibly rename those functions to explicitly mention triple
• possibly add slightly more generic versions, not tied to triples, so that all our Test.make{,_neg} can be replaced by these wrappers
• maybe share the code between the make functions.

[jmid]

Cool! I was thinking about this yesterday too 👍

Logs is a nice and popular library for this kind of logging: https://erratique.ch/software/logs (sans dependencies).
AFAIU, it works through callbacks to a 'reporter'. By default, a "nop-reporter" does nothing and hence is relatively cheap.
To have things printed, one then just need to set another reporter.
I was wondering whether it would be handy to use for us? 🤔