Create a helper library and samples for code checks

[salikh]

Writing autograding tests for student submissions is not a trivial task, as there are unlimited number of ways how the submission may be incorrect. For best student experience, it is necessary to detect many of the possible cases and provide as specific feedback message as possible.

To facilitate creation of good autograder tests, we need to have

• A python library with functions that can do common checks (e.g. presence/type of a variable or a function)
• A collection of code snippets with explanations.