Add tests in `CountFriendsPairing.java`

[Hardvan]

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• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.87%. Comparing base (4bcab89) to head (7f67e98).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5643      +/-   ##
============================================
+ Coverage     58.86%   58.87%   +0.01%     
  Complexity     3768     3768              
============================================
  Files           556      556              
  Lines         15951    15951              
  Branches       3040     3040              
============================================
+ Hits           9389     9391       +2     
+ Misses         6157     6155       -2     
  Partials        405      405              

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[Hardvan]

Thanks!