Add tests, remove main, add negativity test in Fibonacci.java

[Hardvan]

• 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 59.36%. Comparing base (4a0e46d) to head (2a143dc).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5645      +/-   ##
============================================
+ Coverage     59.14%   59.36%   +0.21%     
- Complexity     3782     3797      +15     
============================================
  Files           556      556              
  Lines         15927    15927              
  Branches       3038     3040       +2     
============================================
+ Hits           9420     9455      +35     
+ Misses         6104     6069      -35     
  Partials        403      403              

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