test: `CountCharTest`

[alxkm]

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Added tests.

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

Project coverage is 49.71%. Comparing base (b2815db) to head (a335bcc).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5423   +/-   ##
=========================================
  Coverage     49.71%   49.71%           
- Complexity     3084     3085    +1     
=========================================
  Files           524      524           
  Lines         15171    15173    +2     
  Branches       2884     2884           
=========================================
+ Hits           7542     7544    +2     
  Misses         7314     7314           
  Partials        315      315           

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