Test caching, pytest plugin

[tony]

Multiple, reuseable data seeds

• Downloads initial unihan.zip and reuses it
• Persistent starter seeds which exist between tests, faster reruns
• Creation of starter seeds / orthogonality - simulate a downloaded zip that's not extracted, a complete setup, etc.
• Keeps the fast starter with a small sample of data

[codecov[bot]]

Codecov Report

Merging #291 (3a32f83) into master (0f7d9dd) will increase coverage by 0.00%. The diff coverage is 67.37%.

:exclamation: Current head 3a32f83 differs from pull request most recent head b6f274b. Consider uploading reports for the commit b6f274b to get more accurate results

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   70.39%   70.39%           
=======================================
  Files          12       13    +1     
  Lines        1064     1152   +88     
  Branches      193      216   +23     
=======================================
+ Hits          749      811   +62     
- Misses        287      317   +30     
+ Partials       28       24    -4     

Impacted Files	Coverage Δ
src/unihan_etl/pytest_plugin.py	35.76% <42.50%> (+7.38%)	:arrow_up:
tests/constants.py	100.00% <100.00%> (ø)
tests/test_expansion.py	98.52% <100.00%> (ø)
tests/test_pytest_plugin.py	100.00% <100.00%> (ø)
tests/test_unihan.py	96.55% <100.00%> (ø)

... and 2 files with indirect coverage changes

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