DM-48940: Modify DEBUG logging test to be more robust

[timj]

Rather than assuming that log message N is a debug message, check that any debug messages appeared when debug logging was enabled. This works around issues where CLI plugins could inject their own log messages.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.49%. Comparing base (e3be3c4) to head (b9ebac3).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1144      +/-   ##
==========================================
- Coverage   89.49%   89.49%   -0.01%     
==========================================
  Files         366      366              
  Lines       49020    49018       -2     
  Branches     5941     5941              
==========================================
- Hits        43872    43870       -2     
  Misses       3733     3733              
  Partials     1415     1415              

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