Add Tests for HillCipher.java

[bennybebo]

• [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

Refactored HillCipher.java to remove Scanner dependency and make it testable

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.09091% with 6 lines in your changes missing coverage. Please review.

Project coverage is 53.96%. Comparing base (389d1d7) to head (deeddd2).

Files with missing lines	Patch %	Lines
...ain/java/com/thealgorithms/ciphers/HillCipher.java	89.09%	3 Missing and 3 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5562      +/-   ##
============================================
+ Coverage     53.45%   53.96%   +0.50%     
- Complexity     3365     3385      +20     
============================================
  Files           534      534              
  Lines         15453    15413      -40     
  Branches       2950     2942       -8     
============================================
+ Hits           8261     8317      +56     
+ Misses         6843     6746      -97     
- Partials        349      350       +1     

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