ttl,test: fix the datarace in TestShrinkScanWorkerTimeout

[YangKeao]

What problem does this PR solve?

Issue Number: close #57876

Problem Summary:

The worker has started before it sets the context. It may cause data race.

What changed and how does it work?

Set the context before start the worker.

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.
    It only modified tests.

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

[YangKeao]

/check-issue-triage-complete

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-02 08:28:42.77446971 +0000 UTC m=+1057110.394124226: ☑️ agreed by hawkingrei.
• 2024-12-02 08:30:06.762597091 +0000 UTC m=+1057194.382251606: ☑️ agreed by lcwangchao.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.4529%. Comparing base (6d74071) to head (2715df2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #57883        +/-   ##
================================================
+ Coverage   72.8728%   73.4529%   +0.5801%     
================================================
  Files          1677       1677                
  Lines        464634     464677        +43     
================================================
+ Hits         338592     341319      +2727     
+ Misses       105154     102570      -2584     
+ Partials      20888      20788       -100     

Flag	Coverage Δ
integration	43.3818% <ø> (?)
unit	72.2750% <ø> (+0.0006%)	⬆️

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

Components	Coverage Δ
dumpling	52.7673% <ø> (ø)
parser	∅ <ø> (∅)
br	45.7257% <ø> (+0.0090%)	⬆️