test: add comprehensive unit tests for RaftRegistryServiceImpl

[WangzJi]

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• [x] I have registered the PR changes.

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 71.37%. Comparing base (b27e6be) to head (897518f).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7779      +/-   ##
============================================
+ Coverage     71.03%   71.37%   +0.34%     
- Complexity      797      847      +50     
============================================
  Files          1294     1294              
  Lines         49528    49528              
  Branches       5873     5873              
============================================
+ Hits          35182    35351     +169     
+ Misses        11437    11255     -182     
- Partials       2909     2922      +13     

see 9 files with indirect coverage changes

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