Introduce `@pytest.mark.threadunsafe` to disable thread-based parallelism for specific tests

[ogrisel]

Some tests can be fundamentally not amenable for parallel execution. For instance:

• threads that use the capsys fixture to inspect what is written to sys.stdout and sys.stderr;
• tests that use pytest.warns or more generally fiddle with the non-thread-safe warnings module of the standard library;
• tests that rely on monkeypatching (for instance, via the monkeypatch fixture of unittest.mock);
• ...

It would be nice to have a standard way to mark such tests as unexpected to pass in a thread-based parallel execution setting, and have plugins such as pytest-freethreaded or pytest-run-parallel respect that marker to run them sequentially instead.

This proposal stems from this discussion on the scikit-learn issue tracker with pytest-run-parallel developer @andfoy.

[ogrisel]

For convenience, the runner itself could alternatively detect known fixture usage and code pattern (usage of pytest.warns) that are fundamentally not thread-safe and automatically run those tests sequentially.