tests: make TestScatterRegion stable

[lhy1024]

What problem does this PR solve?

Issue Number: Close #7109

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.21%. Comparing base (dcb426b) to head (315a604).
Report is 1428 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9040      +/-   ##
==========================================
+ Coverage   75.54%   76.21%   +0.66%     
==========================================
  Files         332      467     +135     
  Lines       33892    71113   +37221     
==========================================
+ Hits        25604    54197   +28593     
- Misses       6082    13524    +7442     
- Partials     2206     3392    +1186     

Flag	Coverage Δ
unittests	76.21% <ø> (+0.66%)	⬆️

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[okJiang]

Can you explain why this modification makes the test stable? Thanks~

[lhy1024]

Can you explain why this modification makes the test stable? Thanks~

After clientTestSuite.SetupTest add ResetRegionCache, the test became unstable

I added some logs and found there is no region in region tree when scatter.

So I retry region heartbeat and scatter when meet error