Fix flaky StreamingDataflowWorkerTest which wasn't waiting for enough commits

[scwhittle]

The test waited for 1 commit but expected and validated the 2 commits.
It would pass if both commits happened to be present and fail otherwise.

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R: @chamikaramj added as fallback since no labels match configuration

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.19%. Comparing base (a1a2283) to head (4ae560d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #31781   +/-   ##
=========================================
  Coverage     71.19%   71.19%           
  Complexity     3019     3019           
=========================================
  Files          1058     1058           
  Lines        134159   134159           
  Branches       3257     3257           
=========================================
  Hits          95521    95521           
  Misses        35500    35500           
  Partials       3138     3138           

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