Add tests for `IIRFilter.java`, fix bug in `setCoeffs` method

[Hardvan]

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• [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 56.42%. Comparing base (fa7d357) to head (7f8a7b3).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5590      +/-   ##
============================================
+ Coverage     56.22%   56.42%   +0.19%     
- Complexity     3556     3565       +9     
============================================
  Files           544      544              
  Lines         15693    15693              
  Branches       2985     2985              
============================================
+ Hits           8824     8854      +30     
+ Misses         6494     6463      -31     
- Partials        375      376       +1     

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[Hardvan]

Thanks!