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 54.31%. Comparing base (87871ef) to head (fb4557e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5555      +/-   ##
============================================
+ Coverage     53.96%   54.31%   +0.34%     
- Complexity     3387     3400      +13     
============================================
  Files           535      535              
  Lines         15429    15429              
  Branches       2942     2942              
============================================
+ Hits           8327     8381      +54     
+ Misses         6749     6693      -56     
- Partials        353      355       +2     

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