Add tests for `EulerMethod`

[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

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.49%. Comparing base (9eff71b) to head (c37f4a5).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5769      +/-   ##
============================================
+ Coverage     66.37%   66.49%   +0.11%     
- Complexity     4438     4446       +8     
============================================
  Files           606      606              
  Lines         16850    16850              
  Branches       3242     3242              
============================================
+ Hits          11184    11204      +20     
+ Misses         5220     5200      -20     
  Partials        446      446              

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