Remove mypy error codes from the test_cases directory

[AlexWaygood]

This PR removes mypy error codes from the test_cases directory, and alters mypy_test so that it no longer enforces that type: ignores in the test_cases directory have mypy error codes.

When I created the first test cases, it seemed like a good idea to enforce mypy error codes, as we use mypy error codes in most of the rest of typeshed. But type: ignores in the test_cases directory serve a different purpose to those in the rest of typeshed -- whereas in the rest of typeshed we use them to stop a type checker complaining where we're doing something it doesn't quite understand, in the test_cases directory we use them to signify "this code is meant to fail a type checker". As a result, whereas the error codes in the rest of typeshed help make the stubs more self-documenting, it feels like the error codes in the test_cases directory just end up adding noise. They're also mypy-specific, and, in general, we should probably aim to have as little stuff in typeshed as possible that's specific to one type checker.

More discussion here: #8066 (comment)