test: print helpful err msg on test-dns-ipv6.js

The test sometimes fail on an assertion but no useful error message
was generated for debugging. Modify the test to generate useful
debugging message.

PR-URL: https://github.com/nodejs/node/pull/3501


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>