Suggestion: use pytest-xdist to speed up pytest

[SubaruArai]

Currently, the CI and unit testing uses pytest, which runs the tests sequentially. Here's a brief look of what we can expect if we run pytest concurrently using pytest-xdist:

Potential speedup

test condition	GHA (2vCPU)	i5-8265U with 4vCPU
1 core	3:30	2:21
n cores	2:01	1:02

Drawbacks

The text output will change, and -s option (stdout capture) will not work. As a consequence, log folding will not work, either.

[SubaruArai]

If we really want to keep the full output and folding, we can add pytest-xdist to requirements.txt, and only change the docs for local testing.