[saghm]

Real-world problems tend also not to have existing comprehensive test suites that you can just instantly verify your implementation with. Writing the tests is not only something that usually has to be done by the same individual or team writing the implementation, but also can suffer from the same pitfalls as writing the implementation (incorrect assumptions, missed cases, simply doing the wrong thing, etc.)

[Jensson]

The test suites in competitive programming is a crutch, you aren't supposed to rely on them. Using them to guide your solution is like using the same data to train and test your machine learning model, if you do that then there is no point in practicing.