Add tests for ClosestPair.java

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• 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 ✅

Project coverage is 55.41%. Comparing base (9ce9443) to head (52f11d0).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5555      +/-   ##
============================================
+ Coverage     55.05%   55.41%   +0.35%     
- Complexity     3477     3489      +12     
============================================
  Files           539      539              
  Lines         15607    15607              
  Branches       2965     2965              
============================================
+ Hits           8593     8648      +55     
+ Misses         6656     6599      -57     
- Partials        358      360       +2     

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[siriak]

Looks good, thanks!

[Hardvan]

Thanks!