Under src directory you will find examples of bad program designs. Every example dir is a unique problem. Your task is refactor the code to make it testable and cover the class (or part of the class) with tests.
Imagine that these classes are part of a big and complex application so you are not allowed to change the signatures of methods nor the interface of a class because somewhere it will break the application, but if needed you can add new classes and methods.
If you would like to try to solve these problems, feel free to fork the repo and send me the link in email.
Happy coding!