Refactor case runner logic

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.77%. Comparing base (1aa54a8) to head (3e85381).

Files	Patch %	Lines
foamlib/_cases.py	93.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #135 +/- ## ========================================== - Coverage 89.65% 88.77% -0.88% ========================================== Files 9 9 Lines 812 793 -19 ========================================== - Hits 728 704 -24 - Misses 84 89 +5 ```

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