chore: document testing flow

[dmcilvaney]

Was trying to run the unit tests while experimenting with something, and it was a bit of a trial-and-error type of thing.

I think I captured the steps I went through and added them to the readme, along with creating a requirements file for development dependencies.

The tests should also show up properly in VSCode now.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.08%. Comparing base (8077ac1) to head (e2c8a28).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
- Coverage   85.35%   84.08%   -1.27%     
==========================================
  Files          21       21              
  Lines         867      867              
==========================================
- Hits          740      729      -11     
- Misses        127      138      +11     

Flag	Coverage Δ
integration	63.66% <ø> (ø)
slow-other	?
unit	75.20% <ø> (ø)

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see 3 files with indirect coverage changes

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