Add tests, remove `main`, fix bug in `Sparsity`

[Hardvan]

• [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

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.57%. Comparing base (7e11e9b) to head (8d799c6).

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/misc/Sparsity.java	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5780      +/-   ##
============================================
+ Coverage     66.48%   66.57%   +0.09%     
- Complexity     4445     4451       +6     
============================================
  Files           606      606              
  Lines         16850    16839      -11     
  Branches       3242     3240       -2     
============================================
+ Hits          11202    11211       +9     
+ Misses         5201     5181      -20     
  Partials        447      447              

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[alxkm]

@Hardvan to merge this, update issues should be resolved

[Hardvan]

@alxkm I have resolved the merge conflict.