Try pytest-xdist and pytest-rerunfailures

[mscheltienne]

Closes #228

Locally, test run fine and rarely require retries.
On CIs, some tests are flakky. Let's mark those with pytest-rerunfailures.
But to do so, those test need to be run by workers through xdist to recover from e.g. segfaults.
But all tests can not be run through xdist, e.g. stream resolution.

Plan:

• Define pytest.mark.no_xdist for serial execution
• Mark test which should not be run in parallel
• In CIs, edit the pytest step to run once the test which are not marked with no_xdist with the arguments -n logical --dist worksteal
• In CIs, edit the pytest step to run once the test which are marked with no_xdist

Deprecation in pytest-cov due to pytest-dev/pytest-cov#623

[mscheltienne]

For now, not worth the implementation work.

[mscheltienne]

Some of the test changes in this PR have been merged through #231