planner: fix goroutine leak problem in some unit tests (#26500)

[ti-srebot]

cherry-pick #26500 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26506

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/26506:release-5.1-591ebdd92636

---

What problem does this PR solve?

Issue Number: close #25014, #25190

Problem Summary: planner: fix goroutine leak problem in some unit tests

What is changed and how it works?

planner: fix goroutine leak problem in some unit tests

Check List

Tests

• Unit test

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

@qw4990 you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• wjhuang2016

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[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: ab86672

[ti-chi-bot]

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