General testing code clean-ups

[garyttierney]

A WIP to clean up some of the current test code. This patch also introduces support for reading from the game cache during plugin and integration tests. This shouldn't be an issue for most contributors, who can test against their own cache. However, we are unable to distribute the same cache we are using due to copyright restrictions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 23.29%. Comparing base (11cee7e) to head (a5a369d).
⚠️ Report is 8 commits behind head on kotlin-experiments.

Additional details and impacted files

@@                  Coverage Diff                  @@
##             kotlin-experiments     #434   +/-   ##
=====================================================
  Coverage                 23.29%   23.29%           
  Complexity                  809      809           
=====================================================
  Files                       655      655           
  Lines                     11343    11343           
  Branches                   1591     1591           
=====================================================
  Hits                       2642     2642           
  Misses                     8446     8446           
  Partials                    255      255           

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