chore(ut): add some uts for tiflashgroup and pd

[liubog2008]

• add uts for tiflashgroup
• add uts for common

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 53.07%. Comparing base (1dbb5b1) to head (c3bd82c).

Additional details and impacted files

@@              Coverage Diff               @@
##           feature/v2    #6023      +/-   ##
==============================================
+ Coverage       50.57%   53.07%   +2.49%     
==============================================
  Files             154      154              
  Lines            9459     9445      -14     
==============================================
+ Hits             4784     5013     +229     
+ Misses           4675     4432     -243     

Flag	Coverage Δ
unittest	53.07% <0.00%> (+2.49%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[ti-chi-bot]

@csuzhangxc: 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: csuzhangxc

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