use coverage instead of pytest-cov

[jakkdl]

#2628 replaces trio/_tests/conftest.py and trio/_core/tests/conftest.py with a pytest plugin trio/_tests/pytest_plugin.py that's parsed on startup with -p. This means that the plugin, or anything it imports (i.e. trio.testing) does not get "seen" by pytest-cov, as that's loaded later.

https://stackoverflow.com/a/62224494 suggest using coverage -m pytest [...] instead of pytest --cov [...] to launch coverage before running pytest, which solves this. I started trying to fix it in 00fe706 but decided to postpone it for a separate PR.

[jakkdl]

@A5rocks since you played around with the coverage, could you implement this as well?