balancer/randomsubsetting: Extend the unit tests in the randomsubsetting package.

[marek-szews]

Add more unit tests to increase test coverage of 'randomsubsetting' package.

RELEASE NOTES:

• balancer/randomsubsetting: Implementation of additional UT.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.36%. Comparing base (2d51986) to head (a2f51bc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8781      +/-   ##
==========================================
- Coverage   83.47%   83.36%   -0.11%     
==========================================
  Files         419      419              
  Lines       32595    32595              
==========================================
- Hits        27208    27174      -34     
- Misses       4017     4040      +23     
- Partials     1370     1381      +11     

see 22 files with indirect coverage changes

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