Add UT for maintenance's DeleteOldJobs function

[blackpiglet]

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Does your change fix a particular issue?

Fixes #8593

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• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.31%. Comparing base (b0b5cc4) to head (50ec895).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9013      +/-   ##
==========================================
+ Coverage   60.29%   60.31%   +0.01%     
==========================================
  Files         378      378              
  Lines       42729    42729              
==========================================
+ Hits        25763    25771       +8     
+ Misses      15417    15404      -13     
- Partials     1549     1554       +5     

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[blackpiglet]

After discussion with @Lyndon-Li, we think the original issue was already fixed in v1.16.0's maintenance job refactor PR.
As a result, I will not modify the maintenance job deletion logic anymore.