etcd (ticdc): fix a data race in unit test

[asddongmen]

What problem does this PR solve?

Issue Number: close #4549

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

• None

Side effects

• None

Related changes

• Need to cherry-pick to the release branch

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ben1009

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[codecov-commenter]

Codecov Report

Merging #4551 (f7085d9) into master (9607554) will decrease coverage by 0.0064%.
The diff coverage is 59.5223%.

Flag	Coverage Δ
cdc	60.3340% <64.6700%> (+0.4118%)	⬆️
dm	51.6355% <51.6975%> (-0.3934%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

@@               Coverage Diff                @@
##             master      #4551        +/-   ##
================================================
- Coverage   55.6402%   55.6337%   -0.0065%     
================================================
  Files           494        503         +9     
  Lines         61283      62516      +1233     
================================================
+ Hits          34098      34780       +682     
- Misses        23750      24274       +524     
- Partials       3435       3462        +27     

[ti-chi-bot]

@zhaoxinyu: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[asddongmen]

/merge

[ti-chi-bot]

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

Commit hash: f7085d9

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4576.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4577.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4578.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4579.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4580.