let runTrafficTest polling retry on mismatch

[jukie]

What this PR does / why we need it:

runTrafficTest was calling t.FailNow() whenever compareFunc returned false, which aborted the subtest before wait.PollUntilContextTimeout could retry. The BackendUtilizationLoadBalancing warmup subtest expects retries while Envoy's EDS cluster converges to healthy endpoints, so a single empty trafficMap on the first iteration would fail the test outright. Return false instead and move the diagnostic cluster dump into each caller's poll-timeout branch so it runs once on real failure rather than on every retry.

Fixes #8826

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.61%. Comparing base (f394346) to head (78ffdc5).
⚠️ Report is 171 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8830      +/-   ##
==========================================
- Coverage   73.64%   73.61%   -0.03%     
==========================================
  Files         246      246              
  Lines       49076    49109      +33     
==========================================
+ Hits        36140    36154      +14     
- Misses      10916    10930      +14     
- Partials     2020     2025       +5     

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[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!