benchmark crud: suppress compact warnings during benchmark runs, fixes #9365

[mr-raj12]

Summary

• Suppress repeated Done. Run "borg compact" to free space. warnings that clutter borg benchmark crud output
• measurement_run() calls do_delete() 3 times per test sample, each emitting the warning — 18 extra lines with the default 6 samples
• For --json-lines mode, non-JSON warnings contaminate the JSON stream
• Fix: temporarily raise the borg.archiver logger level to ERROR during measurement runs, restoring it afterward via try/finally
• Single-file change in benchmark_cmd.py, no interface changes

Closes #9365

Checklist

• PR is against master
• Tests pass
• Locally verified: pytest benchmark_cmd_test.py (4/4 passed) and borg benchmark crud --json-lines produces clean JSON-only output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.44%. Comparing base (1150b8d) to head (c7261ae).
⚠️ Report is 12 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9366      +/-   ##
==========================================
- Coverage   76.44%   76.44%   -0.01%     
==========================================
  Files          85       85              
  Lines       14792    14806      +14     
  Branches     2211     2213       +2     
==========================================
+ Hits        11308    11318      +10     
- Misses       2807     2810       +3     
- Partials      677      678       +1     

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[ThomasWaldmann]

LGTM!

[ThomasWaldmann]

Can you please check whether we have the same issue in 1.4-maint?