br: fix flaky test TestStoreRemoved (#52834)

[ti-chi-bot]

This is an automated cherry-pick of #52834

What problem does this PR solve?

Issue Number: close #52791, close #52792

Problem Summary:
When dropping a subscription manager, the pending events will be dropped. So the test case will fail.

What changed and how does it work?

Wait until it seems there isn't new events.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

> go test --count=100 -run '^TestSomeOfStoreUnsupported|TestStoreRemoved$' github.com/pingcap/tidb/br/pkg/streamhelper
ok      github.com/pingcap/tidb/br/pkg/streamhelper     40.463s

• 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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3pointer, YuJuncen

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Needs approval from an approver in each of these files:

• br/OWNERS [3pointer,YuJuncen]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-11 07:31:33.472299445 +0000 UTC m=+515590.707533558: ☑️ agreed by YuJuncen.
• 2024-07-11 07:33:33.174469283 +0000 UTC m=+515710.409703398: ☑️ agreed by 3pointer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-7.5@1ad607b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #54572   +/-   ##
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  Coverage               ?   71.9854%           
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  Files                  ?       1412           
  Lines                  ?     410940           
  Branches               ?          0           
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  Hits                   ?     295817           
  Misses                 ?      95228           
  Partials               ?      19895           

Flag	Coverage Δ
unit	71.9854% <ø> (?)

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	53.4554% <0.0000%> (?)