Clean up arbitrary sleep in GrpcBrokerClusterIntegrationTest

[dttung2905]

I came across this while trying to get familarise with the code base.

Parf ot the #7839

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cad8c56) 61.66% compared to head (38c9c5b) 61.72%.
Report is 31 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12379      +/-   ##
============================================
+ Coverage     61.66%   61.72%   +0.06%     
+ Complexity     1150      207     -943     
============================================
  Files          2426     2426              
  Lines        132776   132776              
  Branches      20541    20541              
============================================
+ Hits          81874    81959      +85     
+ Misses        44884    44804      -80     
+ Partials       6018     6013       -5     

Flag	Coverage Δ
custom-integration1	<0.01% <ø> (ø)
integration	<0.01% <ø> (ø)
integration1	<0.01% <ø> (ø)
integration2	0.00% <ø> (?)
java-11	61.69% <ø> (+0.03%)	⬆️
java-21	61.58% <ø> (+33.87%)	⬆️
skip-bytebuffers-false	61.71% <ø> (+0.05%)	⬆️
skip-bytebuffers-true	61.57% <ø> (+33.86%)	⬆️
temurin	61.72% <ø> (+0.06%)	⬆️
unittests	61.72% <ø> (+0.06%)	⬆️
unittests1	46.88% <ø> (+0.05%)	⬆️
unittests2	27.73% <ø> (+0.02%)	⬆️

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[Jackie-Jiang]

This is incorrect. We need to wait for all segments to be loaded. @dttung2905 @xiangfu0