*: make TestPrimaryChange stable

[rleungx]

What problem does this PR solve?

Issue Number: Close #7237.

What is changed and how does it work?

After the server starts, there will be a time window before starting schedulers.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024

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[codecov]

Codecov Report

Merging #7412 (ae88c29) into master (20da1c7) will decrease coverage by 0.12%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7412      +/-   ##
==========================================
- Coverage   74.28%   74.17%   -0.12%     
==========================================
  Files         451      451              
  Lines       49174    49174              
==========================================
- Hits        36528    36473      -55     
- Misses       9417     9461      +44     
- Partials     3229     3240      +11     

Flag	Coverage Δ
unittests	74.17% <ø> (-0.12%)	⬇️

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[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: ae88c29