test: add unit tests for saga-engine low coverage components

[WangzJi]

• I have read the CONTRIBUTING.md guidelines.
• 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.18%. Comparing base (6160c7d) to head (b2954c3).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7915      +/-   ##
============================================
+ Coverage     71.58%   72.18%   +0.59%     
  Complexity      883      883              
============================================
  Files          1294     1294              
  Lines         49554    49554              
  Branches       5884     5884              
============================================
+ Hits          35475    35772     +297     
+ Misses        11155    10854     -301     
- Partials       2924     2928       +4     

see 19 files with indirect coverage changes

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