Document `net-trace_` file naming scheme.

[afck]

When I run a test with HBBFT_TEST_TRACE=true, it produces ten net-trace_*.txt files with cryptic numbers. We should document in tests/README.md which of them is the last one (corresponding to the simplest test)?

The files even have the same size; is proptest running the exact same test ten times for some reason?

[mbr]

Yes, proptest will run the same test every time if it is a "known regression", stored in tests/net_dynamic_hb.proptest-regressions for example. Running with an identical random seed and unchanged test will result in creating the same network packets as a result.

The "strange numbers" are the process id and a random number, but I will concede that these could be documented better. I did not include a timestamp because the filesystem manages those already sufficiently well.

On the other hand, I am thinking of moving the networking logs to logging and just have a simple, filterable logging facility. How do we want to go ahead here? Add the docs or migrate to logging and just "wait it out"?

[afck]

Migrating to logging would also have the advantage of being able to see the production code's log messages in the right place in the test's log.