HashLoadBalancer unit test

[iwangjie]

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #4383 (43f4e6c) into master (740eaa6) will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4383      +/-   ##
============================================
+ Coverage     68.01%   68.05%   +0.03%     
  Complexity     7563     7563              
============================================
  Files          1029     1029              
  Lines         29372    29372              
  Branches       2652     2652              
============================================
+ Hits          19978    19989      +11     
+ Misses         7812     7801      -11     
  Partials       1582     1582              

see 5 files with indirect coverage changes

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