expression: make test TestPreparePlanCacheOnCachedTable more stable

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #33314

Problem Summary:

What is changed and how it works?

This test is not quite stable in our CI environment... maybe load table data is slow
Add a short sleep to make the code more stable.

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• qw4990

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/8f81cad15ed2467f25328697006ebeffc7ba3423

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 69663e8

[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: 2b0bd62

[hawkingrei]

/run-mysql-test
/run-unit-test

[tiancaiamao]

/run-unit-test

[ti-srebot]

cherry pick to release-6.0 in PR #33355