Make functional tests faster

[agateau-gg]

Context

Our test suites can run faster by making use of the pytest-xdist plugin, which can distribute tests on several cores.

What has been done

• Install pytest-xdist
• Make our functional test suite use it
• Fix some pytest-xdist-related issues in our unit-test suite

Unfortunately this PR does not activate pytest-xdist on our unit-test suite for now because some tests randomly fail: sometimes log messages are not received when they should be (that could be related to the recent logging work I did 😒) and sometimes SecretScanner tests fail because of a corrupted .cache_ggshield file.

Validation

Run make functest. It should be faster.

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.04%. Comparing base (035d74b) to head (193862c).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1017   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         181      181           
  Lines        7717     7717           
=======================================
  Hits         7103     7103           
  Misses        614      614           

Flag	Coverage Δ
unittests	92.04% <ø> (ø)

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