Add unit tests to mypy targets; fix typing in tests (part of #116)

[binaryDiv]

This is a follow up PR to #120, which solves another part of #116.

It adds the unit tests to the targets of mypy and fixes typing issues in those files.

There are two rules that are ignored for now:

1. Untyped definitions are allowed in the unit tests, e.g. you don't need to specify parameter types and the return type for test functions (def test_something(input_data): is sufficient). This is because a) the return type is always None anyway, and b) for a lot of the test parameters it would make sense to use Any anyway.
2. assignment errors are ignored. This is the main issue of using validataclass and mypy in a project. Defining a validataclass with e.g. foo: int = IntegerValidator() will result in a type error, because mypy thinks we assign an IntegerValidator object to an attribute that has the type int. To solve this, we probably need a custom mypy-validataclass integration, which will be the main and last part of Support for mypy #116.