Run unit tests in parallel

[Kami]

This PR attempts to run unit tests in parallel using pytest-xdist plugin.

To avoid issues with some of the test functions not being "cross process safe" we run all the tests in a single test file in the same worker process.

Locally (with 16 cores) unit tests now finish ~5-10x faster, but it depends on number of available cores.

[codecov-commenter]

Codecov Report

Merging #1625 (b0eeb3f) into trunk (748dc59) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##            trunk    #1625      +/-   ##
==========================================
- Coverage   83.16%   83.15%   -0.01%     
==========================================
  Files         397      397              
  Lines       86545    86542       -3     
  Branches     9197     9197              
==========================================
- Hits        71979    71968      -11     
- Misses      11436    11444       +8     
  Partials     3130     3130              

Impacted Files	Coverage Δ
libcloud/test/test_utils.py	95.18% <ø> (-0.13%)	⬇️
libcloud/test/storage/test_local.py	91.51% <50.00%> (+0.02%)	⬆️
libcloud/test/loadbalancer/test_brightbox.py	91.04% <100.00%> (+0.13%)	⬆️
libcloud/utils/connection.py	11.11% <0.00%> (-88.89%)	⬇️

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[Kami]

This change uncovered some issues with some test files relying on shared state (aka when running all the test cases together in the same process some other import unintentionally setting some state which test relies on) which I'm trying to fix.

[Kami]

On CI, tests now finish 30-40 seconds faster (2 min 20 vs 1 min 40). Not a massive speed up, but GHA runner likely doesn't have as many cores and it's likely more heavily over provisioned with other jobs.