test: check return address family when detecting IPv6

According to our documentation:

> The callback has arguments (err, address, family). The address
> argument is a string representation of a IP v4 or v6 address. The
> family argument is either the integer 4 or 6 and denotes the family of
> address (not necessarily the value initially passed to lookup).

This fixes this test on CentOS 5.10.