test: apply Linux workaround to Linux only

Do not try Ubuntu/SUSE/Debian-specific hostnames for IPv6 localhost
unless we are on Linux.

Fixes: https://github.com/nodejs/node/issues/4546
PR-URL: https://github.com/nodejs/node/pull/5471


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>