net: disable IPv6 multicast tests unless external IPv6 connection is present
net: disable IPv6 multicast tests unless external IPv6 connection is present

R=rsc
CC=golang-dev
http://codereview.appspot.com/6398052
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go