test: LongestPalindromicSubstring

[alxkm]

Added test LongestPalindromicSubstringTest

• 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 48.86%. Comparing base (7674a84) to head (2c30cb4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5402      +/-   ##
============================================
+ Coverage     48.72%   48.86%   +0.14%     
- Complexity     3025     3037      +12     
============================================
  Files           524      524              
  Lines         15266    15255      -11     
  Branches       2901     2899       -2     
============================================
+ Hits           7438     7455      +17     
+ Misses         7518     7493      -25     
+ Partials        310      307       -3     

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