Add -h/--help, -l/--list cmdline args to test main
Add -h/--help, -l/--list cmdline args to test main

Summary:
Also: allow setting default log levels via env var.

Rationale: since quite a few log modules were added recently in FLUFF to make log filtering a bit easier, I needed a way to set default log level when running C tests. And since I was already fiddling with avs_unit code, I realized that a help message could be extremely useful. I don't really have a specific rationale for the `--list`, though.

Test Plan: manual check with fluff tests

Reviewers: kfyatek, mkrawiec

Differential Revision: https://phabricator.avsystem.com/D2813

AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons