test/cluster_test: make TestRegionStatistics more stable (tikv#8320)

[HuSharp]

Signed-off-by: husharp [email protected]<!--

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What problem does this PR solve?

Issue Number: Close #xxx

What is changed and how does it work?

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[HuSharp]

/test pull-integration-realcluster-test

[HuSharp]

/test ?