AcceptHandler closes child channel if parent channel is closed and there's no test for that

[weissi]

https://github.com/apple/swift-nio/blob/39047aec7c5a0738cba2c8381dc7703c5f02b771/Sources/NIOPosix/Bootstrap.swift#L453

This is lacking a test and we should think about whether that's exctly right anyway

[weissi]

As in no unit test hits this line