[WIP] Speed up unit tests

[joeylegere]

This is an interim solution to make our CI wait times less unbearable until we switch to Jenkins. Leverages pytest-split and github actions' matrix strategy to split all our tests across multiple workers.

If there are tests which are not part of the test durations file, then pytest-split assumes average execution time for them. This shouldn't cause too much of a problem, but I've included a script to periodically regenerate these average cases if we want to.

[codecov]

Codecov Report

Merging #760 (134bcf4) into master (ed6a908) will increase coverage by 0.16%.
The diff coverage is n/a.

❗ Current head 134bcf4 differs from pull request most recent head 20f6a97. Consider uploading reports for the commit 20f6a97 to get more accurate results

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   81.86%   82.02%   +0.16%     
==========================================
  Files          42       42              
  Lines        4984     4984              
==========================================
+ Hits         4080     4088       +8     
+ Misses        904      896       -8     

Impacted Files	Coverage Δ
bittensor/_dataset/dataset_impl.py	89.40% <0.00%> (-0.32%)	⬇️
bittensor/_subtensor/subtensor_impl.py	79.44% <0.00%> (ø)
bittensor/_metagraph/metagraph_impl.py	64.02% <0.00%> (+0.30%)	⬆️
bittensor/_threadpool/__init__.py	80.95% <0.00%> (+4.76%)	⬆️
bittensor/_config/__init__.py	80.64% <0.00%> (+9.67%)	⬆️

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