server: fix tests when tidb-server already listening

[morgo]

What problem does this PR solve?

Issue Number: close #26067

Problem Summary:

The server tests did not permit a tidb-server to already be listening on port 4000. This is a regression of #24991

What is changed and how it works?

What's Changed:

tests only

Related changes

• None

Check List

Tests

• Unit test

Side effects

• None

Release note

• No release note

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[bb7133]

/run-integration-tests

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• tiancaiamao
• wjhuang2016

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[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: 943cee7

[ti-chi-bot]

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/run-all-tests

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