Add test requirement to pull request template

[DenizAltunkapan]

• 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

This PR updates the pull request template to include a checklist item requiring a corresponding test class for every newly added class.

The goal is to improve code quality and ensure better test coverage, as PRs are frequently opened with new implementations but without associated tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (6149ec7) to head (1968ee2).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7185      +/-   ##
============================================
- Coverage     78.91%   78.91%   -0.01%     
  Complexity     6883     6883              
============================================
  Files           774      774              
  Lines         22746    22746              
  Branches       4468     4468              
============================================
- Hits          17951    17949       -2     
- Misses         4076     4077       +1     
- Partials        719      720       +1     

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