statistics: add more test for tidb_merge_partition_stats_concurrency (#48965)

[ti-chi-bot]

This is an automated cherry-pick of #48965

What problem does this PR solve?

Issue Number: close #48713

Problem Summary:

What changed and how does it work?

Check List

Tests

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

  • I checked and no code files have been changed.

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

[hawkingrei]

/test all

[tiprow]

@hawkingrei: No presubmit jobs available for pingcap/tidb@release-7.5

In response to this:

/test all

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, winoros

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-05 14:33:05.055534936 +0000 UTC m=+1538013.720761153: ☑️ agreed by hawkingrei.
• 2023-12-06 08:18:54.915955277 +0000 UTC m=+1601963.581181472: ☑️ agreed by winoros.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.5@069631e). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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##             release-7.5     #49013   +/-   ##
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Flag	Coverage Δ
unit	71.9933% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.2648% <0.0000%> (?)