test: fix test-dns.js flakiness

Use empty string instead of `www.google.com` for tests where we are just
doing parameter evaluation. This will avoid DNS lookups which appear to
be causing flakiness on Raspberry Pi devices in CI.

PR-URL: https://github.com/nodejs/node/pull/5996
Fixes: https://github.com/nodejs/node/issues/5554


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>