test: fix test-net-dns-custom-lookup test assertion

The assertion made an assumption that the IPv6 address would always be
`::1`. Since the address can be different on different platforms, it
has been changed to allow multiple addresses.

Fixes: https://github.com/iojs/io.js/issues/1527
PR-URL: https://github.com/iojs/io.js/pull/1531


Reviewed-By: Rod Vagg <rod@vagg.org>