CI: Speed up test via `pytest-xdist`

[Zeroto521]

Is your feature request related to a problem? Please describe.

All test cases are tested in sequence, not in parallel.
It‘s slower to finish a complete workflow. The average run time of coverage.yml is 3m 54s for last year.

Describe the solution you'd like

Use https://github.com/pytest-dev/pytest-xdist to help us via a simple command pytest -n auto.

[mmghannam]

Interesting, I'd happily accept a PR :)

[jonathanberthias]

I was just working on a very related feature #1082 so I added pytest-xdist as part of it, great suggestion @Zeroto521!