[feature request] support running tests with python-xdist

[Apteryks]

Hello,

Running the test suite with pytest-xdist in parallel reduces its length from 280 s to 80 s. Unfortunately, it introduces a few spurious test failures as spot in #988. It'd be nice to ensure the tests can run safely concurrently to support this use case.

Thanks for your consideration!

[Apteryks]

There appears to be just two test modules which have problematic tests when it comes to running them in parallel, as using the following pytest options I can run the test suite 10 times in a row without fail:

--ignore=tests/test_restarter.py
--ignore=tests/test_client.py