Allow different arg names for positional-only arguments in signature tests

[honno]

Currently the suite fails when signatures use different arg names then whats in the spec for pos-only args, even though it shouldn't really matter.

For example, testing a function with the following signature

def f(x, y, /):
    ...

should pass an implementation of f(a, b, /).

Thanks @Micky774 for catching this!