Implement a unit test with at least four test cases

[Zediwan]

• [x] Create a separate file dedicated to unit tests.
• [x] Write a unit test that tests a function in your code that could produce an unexpected output.
• [x] Provide a description of how this function might produce an unexpected result and how you tested it in a "testing.md" file.

You can implement more that one unit test, but this is optional.