tests/util/test_app: Replace db() with db_conn()

[Turbo87]

The db() function with its closure comes from a time where there was only a single database connection available for each test. This restriction had been lifted quite a while ago, so this is no longer needed and we can just return the connection directly from the function instead.

Review in "hide whitespace changes" mode recommended... 😅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.86%. Comparing base (5f392c3) to head (dbae8d9).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9718      +/-   ##
==========================================
- Coverage   88.91%   88.86%   -0.05%     
==========================================
  Files         288      288              
  Lines       29624    29501     -123     
==========================================
- Hits        26340    26217     -123     
  Misses       3284     3284              

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