docs: Add reference docs for tap & target testing helpers

[edgarrmondragon]

https://meltano-sdk--2219.org.readthedocs.build/en/2219/testing.html#reference

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📚 Documentation preview 📚: https://meltano-sdk--2219.org.readthedocs.build/en/2219/

[codspeed-hq]

CodSpeed Performance Report

Merging #2219 will not alter performance

_{Comparing edgarrmondragon/docs/testing-helpers-reference (c449cea) with main (2f597fe)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2f597fe) 88.47% compared to head (c449cea) 88.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2219   +/-   ##
=======================================
  Coverage   88.47%   88.47%           
=======================================
  Files          54       54           
  Lines        4720     4720           
  Branches      919      919           
=======================================
  Hits         4176     4176           
  Misses        383      383           
  Partials      161      161           

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