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.

[sylvlecl]

Closed in favor of https://github.com/AntaresSimulatorTeam/AntaREST/pull/2133 which fixes naming issues