*: make TestEtcdClientSync stable

[rleungx]

What problem does this PR solve?

Issue Number: Close #8085

What is changed and how does it work?

If we remove the member we connect to, we might see member is stopped before the response arrives.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• nolouch

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[codecov]

Codecov Report

Merging #8084 (c7b1e12) into master (2dd2301) will increase coverage by 0.01%.
Report is 2 commits behind head on master.
The diff coverage is n/a.

❗ Current head c7b1e12 differs from pull request most recent head 23d61ed. Consider uploading reports for the commit 23d61ed to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8084      +/-   ##
==========================================
+ Coverage   77.29%   77.30%   +0.01%     
==========================================
  Files         469      469              
  Lines       61097    61097              
==========================================
+ Hits        47225    47233       +8     
+ Misses      10313    10311       -2     
+ Partials     3559     3553       -6     

Flag	Coverage Δ
unittests	77.30% <ø> (+0.01%)	⬆️

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: 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: c7b1e12

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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