What

This MR adds --mem-warn, which causes Tezt to warn if a test uses too much memory.

Why

This allows to get a better idea of which tests are problematic before a crash.

How

There are two warnings:

one when a test crosses the threshold, so we know there is an issue even if Tezt crashes;
one at the end, to see the actual peak memory usage.

Manually testing the MR

dune exec test/unix/main.exe -- --mem-warn 100000 -j 2 --mem-poll-frequency 10000

Checklist

Update CHANGES.md. No need to document changes to documentation and tests.
Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
Add tests in test/, if relevant.