*: make TestStores stable

[okJiang]

What problem does this PR solve?

Issue Number: Close #7954

What is changed and how does it work?

add a failpoint to make sure the offline store do not convert to tombstone

Check List

Tests

• Unit test

Release note

None.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.62%. Comparing base (a766351) to head (3abf545).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8522      +/-   ##
==========================================
+ Coverage   77.56%   77.62%   +0.05%     
==========================================
  Files         474      474              
  Lines       61861    61877      +16     
==========================================
+ Hits        47981    48030      +49     
+ Misses      10347    10322      -25     
+ Partials     3533     3525       -8     

Flag	Coverage Δ
unittests	77.62% <33.33%> (+0.05%)	⬆️

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[okJiang]

/cc @rleungx @HuSharp

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nolouch, rleungx

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• OWNERS [nolouch,rleungx]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-16 04:24:42.645882544 +0000 UTC m=+501167.349352188: ☑️ agreed by nolouch.
• 2024-08-22 07:52:06.506323377 +0000 UTC m=+424721.640773498: ☑️ agreed by rleungx.