Make `ignore_pos_arg_names=False` for overrides when using `--extra-checks`

Feature

We can check positional argument names for overriding functions.

class A:
    def f(self, x: int) -> None:
        pass
    
class B(A):
    def f(self, _x: int) -> None:  # this *should* error, but doesn't currently
        pass

Pitch

Well, ignoring positional arguments is obviously not type safe. It's annoying, sure, but in my mind that's the class of checks that --extra-checks encompasses (technically correct but annoying).

Start by tracing override checking and the handling of positional argument names when --extra-checks is enabled. Confirm the existing behavior with the A and B example, then add coverage showing that differing positional names are reported under --extra-checks and that the check remains ignored otherwise.