tests: Cache configuration

[tony]

1. Caches tests project-wide (.unihan_cache, .cihai_cache)
2. Caches via scope='session' of pytests

[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 76.09%. Comparing base (e5a5187) to head (8190b17).

❗ Current head 8190b17 differs from pull request most recent head 46d1797. Consider uploading reports for the commit 46d1797 to get more accurate results

Files	Patch %	Lines
tests/test_cli.py	0.00%	3 Missing ⚠️
tests/conftest.py	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
+ Coverage   75.37%   76.09%   +0.72%     
==========================================
  Files           5        5              
  Lines         268      297      +29     
==========================================
+ Hits          202      226      +24     
- Misses         66       71       +5     

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