tests: Fix unstable unit-tests

[JinheLin]

What problem does this PR solve?

Issue Number: close #9309

Problem Summary:

Background tasks may flush MemTableSet of delta before snapshot is created.

What is changed and how it works?

Disable `flushCache` in some unit-tests. 

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, Lloyd-Pottiger

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• OWNERS [JaySon-Huang,Lloyd-Pottiger]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-12 03:46:26.942322487 +0000 UTC m=+153271.645792126: ☑️ agreed by JaySon-Huang.
• 2024-08-12 03:48:52.499119205 +0000 UTC m=+153417.202588833: ☑️ agreed by Lloyd-Pottiger.