test: remove valid hostname check in test-dns.js

Operating systems can and do return invalid hostnames if that's
what they have (for example) in /etc/hosts. Test passes if no
error is thrown and the hostname string is not empty.

Fixes: https://github.com/nodejs/node/issues/2468
PR-URL: https://github.com/nodejs/node/pull/2785


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>