test: LongestAlternatingSubsequenceTest

[alxkm]

Class cleanup.
Readability improvements.
Added tests.

• 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 47.75%. Comparing base (93e4175) to head (fc95deb).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5399      +/-   ##
============================================
+ Coverage     47.64%   47.75%   +0.11%     
- Complexity     2947     2956       +9     
============================================
  Files           524      524              
  Lines         15298    15292       -6     
  Branches       2906     2905       -1     
============================================
+ Hits           7288     7303      +15     
+ Misses         7709     7689      -20     
+ Partials        301      300       -1     

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