refactor: Add more tests in `HashMap`

[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 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.20%. Comparing base (cd40dfb) to head (0ba6382).

Files with missing lines	Patch %	Lines
...rithms/datastructures/hashmap/hashing/HashMap.java	85.71%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5974   +/-   ##
=========================================
  Coverage     71.20%   71.20%           
- Complexity     4893     4896    +3     
=========================================
  Files           650      650           
  Lines         17526    17533    +7     
  Branches       3376     3378    +2     
=========================================
+ Hits          12479    12484    +5     
  Misses         4580     4580           
- Partials        467      469    +2     

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