ci: parallel pytest execution

[sylvlecl]

Experiment the use of pytest-xdist to speed up test execution in CI and dev environment:
see for example this job where tests are executed in under 10min on ubuntu.

Following issues with parallel execution of tests are solved :

• some checks depended on the consistency at 2 moments in time of the result of disk_usage, which was anyway not a good idea (dependent on what happens elsewhere on the machine). It's now replaced with mocking
• some parameterized tests had random sort order, which is not allowed by pytest-xdist (all workers must have exactly the same tests in the same order)
• the command_factory test checked something in its teardown, assuming all tests were executed in the same process, which is not the case anymore. The same check is done as a separate unit tests.

[mabw-rte]

Do you have an idea why this was not corrected by isort ?

[sylvlecl]

Actually there is no change on import sort order in the final PR. Intermediate commits introduced unwanted changes (related to another PR).

[mabw-rte]

@sylvlecl

• For the rebase it should be launched locally (always).
• The commit messages should be corrected.
• The branch naming does not respect the git flow we fixed.

[sylvlecl]

Closed in favor of #2133 which fixes naming issues