chore: add UT for PD/TiKV/TiDB/TiFlash APIs

[csuzhangxc]

What problem does this PR solve?

What is changed and how does it work?

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.55%. Comparing base (749fbbb) to head (5150ebb).

Additional details and impacted files

@@              Coverage Diff               @@
##           feature/v2    #6012      +/-   ##
==============================================
+ Coverage       24.50%   27.55%   +3.04%     
==============================================
  Files             151      151              
  Lines            9912     9912              
==============================================
+ Hits             2429     2731     +302     
+ Misses           7294     6932     -362     
- Partials          189      249      +60     

Flag	Coverage Δ
unittest	27.55% <100.00%> (+3.04%)	⬆️

[ti-chi-bot]

@fgksgf: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: fgksgf

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment