[ISSUE #8665] Add more test coverage for RebalanceLockManager

[yx9o]

Fixes #8665 .

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.49%. Comparing base (e7eda2c) to head (340dcde).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #8666      +/-   ##
=============================================
+ Coverage      47.43%   47.49%   +0.05%     
- Complexity     11517    11531      +14     
=============================================
  Files           1277     1277              
  Lines          89442    89442              
  Branches       11502    11502              
=============================================
+ Hits           42430    42482      +52     
+ Misses         41812    41748      -64     
- Partials        5200     5212      +12     

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