testing: random test ordering

[germa89]

Working on #3257 and #3254, I realized that some tests do affect the tests after them (normally in a bad way).
Although mentioned PRs aims to mitigate that, I think it will be clever to have random test ordering.

Keeping in mind that ideally all the pool tests should run once after another (order independent), so we can start and stop the pool just once.

https://pypi.org/project/pytest-random-order/