Test the is_invalid_named_type function.

[emilio]

In #384 I landed a is_invalid_named_type function.

We should add tests to ensure it doesn't mess it up, it'd be helpful to have a few unit tests or doctests for it.

An example of such a test would look like:

let ty = Type::new(Some("foo".into()), None, TypeKind::Named, false);
assert!(!ty.is_invalid_named_type())

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[fitzgen]

Fixed in cedadf9