Update TestCase.load() to load in place

[tysmith]

• Remove TestCase.purge_optional since this now happens automatically.
• TestCase.load() now only loads a single test case.
• Break up test case loading in to smaller more testable parts.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3e00f32) 99.59% compared to head (4d31459) 99.51%. Report is 3 commits behind head on master.

:exclamation: Current head 4d31459 differs from pull request most recent head 30542a8. Consider uploading reports for the commit 30542a8 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #390 +/- ## ========================================== - Coverage 99.59% 99.51% -0.08% ========================================== Files 50 50 Lines 5481 5408 -73 ========================================== - Hits 5459 5382 -77 - Misses 22 26 +4 ```

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[matt-boris]

generic-worker v58.0.2 should fix this simple engine issue you're seeing on your mac runner.