test: add unique check for mock backend ctx

[tangenta]

What problem does this PR solve?

Issue Number: close #52874

Problem Summary:

[2024/04/26 11:00:14.826 +08:00] [INFO] [mock.go:211] ["mock writer write row"] [key=7480000000000000685f698000000000000002016475706c69636174ff6576616c75650000fd] [idxVal=007d017f001c038000000000000000016475706c69636174ff6576616c75650000fd80000100000002010000]
[2024/04/26 11:00:14.826 +08:00] [INFO] [mock.go:211] ["mock writer write row"] [key=7480000000000000685f698000000000000002016475706c69636174ff6576616c75650000fd] [idxVal=007d017f001c038000000000000001016475706c69636174ff6576616c75650000fd80000100000002010000]

During the backfill step, writing to the same key for unique index should report "key exists" error.

What changed and how does it work?

Add unique check for mock backend ctx.

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

[tiprow]

Hi @tangenta. Thanks for your PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.7528%. Comparing base (1d698c7) to head (a9e3219).
Report is 6 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #52900         +/-   ##
=================================================
- Coverage   72.3811%   55.7528%   -16.6284%     
=================================================
  Files          1482       1596        +114     
  Lines        428743     599021     +170278     
=================================================
+ Hits         310329     333971      +23642     
- Misses        99115     241962     +142847     
- Partials      19299      23088       +3789     

Flag	Coverage Δ
integration	37.0829% <38.0952%> (?)
unit	71.2341% <100.0000%> (+0.0156%)	⬆️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	49.8177% <ø> (+8.6738%)	⬆️

[tangenta]

/retest

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, ywqzzy

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-26 06:56:27.130074908 +0000 UTC m=+329743.869977820: ☑️ agreed by ywqzzy.
• 2024-04-26 07:17:35.01496568 +0000 UTC m=+331011.754868594: ☑️ agreed by lance6716.

[lance6716]

/retest

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[tangenta]

/retest

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[ti-chi-bot]

@tangenta: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-mysql-client-test	a9e3219	link	unknown	/test pull-mysql-client-test
idc-jenkins-ci-tidb/unit-test	a9e3219	link	unknown	/test unit-test

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