ddl: fix unstable TestMDLStaleRead

[tangenta]

What problem does this PR solve?

Issue Number: close #42061

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• wjhuang2016

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[hawkingrei]

/retest

[hawkingrei]

/check-issue-triage-complete

[hawkingrei]

/retest

[hawkingrei]

/check-issue-triage-complete

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 4a3968c

[hawkingrei]

/retest

[hawkingrei]

/test mysql-test

[hawkingrei]

/retest

[ti-chi-bot]

@tangenta: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	4a3968c	link	unknown	/test mysql-test

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