test: Separate TestPdClient from test_raftstore component

[haojinming]

Signed-off-by: haojinming jinming.hao@pingcap.com

What is changed and how it works?

Issue Number: Close #13452

What's Changed:

`TestPdClient` is in `test_raftstore` component, which may cause circular dependency. For example, `causal_ts` need `TestPdClient`  and `test_raftstore` depend on `causal_ts`. 
Separate `TestPdClient` from `test_raftstore` component to avoid circular dependency.

Related changes

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Side effects

Release note

None

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[REVIEW NOTIFICATION]

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• Connor1996
• pingyu

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[haojinming]

/cc @pingyu @zeminzhou PTAL~, thanks.

[zeminzhou]

LGTM~

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[pingyu]

LGTM~

[pingyu]

/merge

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

Commit hash: 004b165