Add tests for AStar.java, enhance documentation

[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 57.80%. Comparing base (d422bf5) to head (11b5bde).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5603      +/-   ##
============================================
+ Coverage     57.44%   57.80%   +0.35%     
- Complexity     3638     3644       +6     
============================================
  Files           545      545              
  Lines         15708    15696      -12     
  Branches       2994     2993       -1     
============================================
+ Hits           9024     9073      +49     
+ Misses         6302     6244      -58     
+ Partials        382      379       -3     

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[siriak]

Looks good, thanks!

[Hardvan]

Thanks!