cherry-pick #1325 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/1339

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

git push git@github.com:ti-srebot/ticdc.git pr/1339:release-4.0-f5d7cfa048de

What problem does this PR solve?

Improve unit test of kv client, part of #1292

What is changed and how it works?

• Test kv client should drop an event if its request-id is outdated
• Test kv client should return an error when receiving a new subscription but the corresponding region is not found in pending regions

Check List

Tests

• Unit test
• Integration test

Release note

• No release note