[Flaky Tests] Avoid re-triggering io-thread activation

[sarthakaggarwal97]

The test was accidentally waking the IO threads while trying to check that they had gone idle.

After the recent IO-thread refactor in #3324, the test started forcing io-threads-always-active, and its repeated INFO polling counted as fresh activity. So instead of just observing the worker threads, the test kept reactivating them and then flaked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.45%. Comparing base (b2d08c9) to head (253f929).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3509      +/-   ##
============================================
+ Coverage     76.40%   76.45%   +0.05%     
============================================
  Files           159      159              
  Lines         79851    79851              
============================================
+ Hits          61008    61053      +45     
+ Misses        18843    18798      -45     

see 25 files with indirect coverage changes

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[sarthakaggarwal97]

Should get stabilized by #3511