executor: migrate test-infra to testify for show_placement_labels_test.go

[feitian124]

What problem does this PR solve?

Issue Number: close #28609

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• xhebox

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[hawkingrei]

/check-issue-triage-complete

[hawkingrei]

/run-unit-test

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/b536902f42cf544962b8885f3e215d6855e30f51

[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: fe9f034

[ti-chi-bot]

@feitian124: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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