br: fix flaky test br_txn

[Tristan1900]

What problem does this PR solve?

Issue Number: close #54979

Problem Summary:

What changed and how does it work?

Feels like the way of generating load might have a race condition with the test setup. We noticed a mismatch of number of items before and after restore causing checksum mismatch.

This PR changes the code to generate a fixed number of items and run locally several times and cannot reproduce the issue anymore.

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

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[tiprow]

Hi @Tristan1900. Thanks for your PR.

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[Tristan1900]

/ok-to-test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.1302%. Comparing base (74034d4) to head (76097f7).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #56444        +/-   ##
================================================
+ Coverage   73.3645%   76.1302%   +2.7656%     
================================================
  Files          1624       1624                
  Lines        448069     448152        +83     
================================================
+ Hits         328724     341179     +12455     
+ Misses        99207      85973     -13234     
- Partials      20138      21000       +862     

Flag	Coverage Δ
integration	50.3584% <100.0000%> (?)
unit	72.5243% <66.6666%> (+0.0606%)	⬆️

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

Components	Coverage Δ
dumpling	52.9478% <ø> (ø)
parser	∅ <ø> (∅)
br	63.0155% <100.0000%> (+17.5025%)	⬆️