tests(cdc): fix data race in TestMessageClientBasics

[liuzix]

What problem does this PR solve?

Issue Number: ref #3825

What is changed and how it works?

• Made mockSendMessageClient thread-safe.

Check List

Tests

• Unit test

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:

• 3AceShowHand
• amyangfei

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[3AceShowHand]

/run-integration-test
/run-kafka-integration-test

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: d9e7dfe

[liuzix]

/run-integration-tests

[ti-chi-bot]

@liuzix: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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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[overvenus]

/run-integration-tests /tidb=pr/30558
/run-kafka-integration-test /tidb=pr/30558

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4428.