kv/client: fix unstable unit test, where request id is not accurate (#1674)

[ti-srebot]

cherry-pick #1674 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In ticdc repo:
git pr https://github.com/pingcap/ticdc/pull/1679

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

git push git@github.com:ti-srebot/ticdc.git pr/1679:release-4.0-7f5a772f9785

---

What problem does this PR solve?

Fix a test failure in https://github.com/pingcap/ticdc/issues/1501#issuecomment-820027078

Note release-5.0 should be cherry-picked manually.

What is changed and how it works?

The cause is that when the mock TiKV server starts up for the second time, it could cost some time before it is available and kv client will try to request for more than one time.

Check List

Tests

• Unit test
• Integration test

Release note

• No release note

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[amyangfei]

I will resolve after #1668 is merged

[amyangfei]

will be cherry-picked in #1668