test: print helpful err msg on test-dns-ipv6.js
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 <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>
Reviewed-By: Rich Trott <28a7b6144c43947e3f532c033ff42c8a2ae08045@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node