tests: close gRPC connections after testing

[rleungx]

What problem does this PR solve?

Issue Number: Close #xxx

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

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[ti-chi-bot]

[FORMAT CHECKER NOTIFICATION]

Notice: To remove the do-not-merge/needs-linked-issue label, please provide the linked issue number on one line in the PR body, for example: Issue Number: close #123 or Issue Number: ref #456, multiple issues should use full syntax for each issue and be separated by a comma, like: Issue Number: close #123, ref #456.

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[ti-chi-bot]

@rleungx: 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-unit-test-next-gen-2	0b33e8e	link	true	/test pull-unit-test-next-gen-2
pull-unit-test-next-gen-3	0b33e8e	link	true	/test pull-unit-test-next-gen-3

Full PR test history. Your PR dashboard.

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[bufferflies]

How about adding a new function as close for pdClient, it will close this conn?