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 72.39%. Comparing base (a1bb731) to head (4419898).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7915      +/-   ##
============================================
+ Coverage     71.74%   72.39%   +0.64%     
  Complexity      872      872              
============================================
  Files          1310     1310              
  Lines         49977    49977              
  Branches       5945     5945              
============================================
+ Hits          35856    36180     +324     
+ Misses        11160    10834     -326     
- Partials       2961     2963       +2     

see 18 files with indirect coverage changes

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