test: Speeds up tests & improves console output

[frgfm]

Tests are starting to be a bottleneck so I added common plugins to pytest:

• xdist: to run tests in parallel (considerable speedups when you have several workers: only 2 on GitHub but it's still twice as fast, and locally it's dozens of times faster). Solely for client for now, as we need to properly handle the multiple connections to the DB for the API test
• cov: integrates coverage in pytest directly
• pretty: better output

Any feedback is welcome!

[codecov[bot]]

Codecov Report

Merging #301 (c9af336) into main (51fcdb3) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #301   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          66       66           
  Lines        1590     1590           
=======================================
  Hits         1506     1506           
  Misses         84       84           

Flag	Coverage Δ
client	90.58% <ø> (ø)
unittests	94.95% <ø> (ø)

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