*: make TestKeyspaceGroupMergeIntoDefault stable

[rleungx]

What problem does this PR solve?

Issue Number: Close #6991.

What is changed and how does it work?

The allocNodesToAllKeyspaceGroups only runs once. If we create keyspace groups by two times, it might lose the latter one.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• JmPotato

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[codecov]

Codecov Report

Merging #8080 (de4d555) into master (4c0a862) will increase coverage by 0.06%.
The diff coverage is n/a.

❗ Current head de4d555 differs from pull request most recent head 41bfd79. Consider uploading reports for the commit 41bfd79 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8080      +/-   ##
==========================================
+ Coverage   77.32%   77.39%   +0.06%     
==========================================
  Files         469      469              
  Lines       61097    61097              
==========================================
+ Hits        47246    47286      +40     
+ Misses      10288    10259      -29     
+ Partials     3563     3552      -11     

Flag	Coverage Δ
unittests	77.39% <ø> (+0.06%)	⬆️

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: de4d555

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

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