test: Fix failing file cache test in CI

[nameexhaustion]

I think the eviction task ran instantly - to fix this we won't check if the paths were created, instead we just check the file is successfully read.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.02%. Comparing base (c19c377) to head (28957d0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17014      +/-   ##
==========================================
- Coverage   81.04%   81.02%   -0.02%     
==========================================
  Files        1446     1446              
  Lines      190422   190422              
  Branches     2708     2708              
==========================================
- Hits       154331   154293      -38     
- Misses      35594    35632      +38     
  Partials      497      497              

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