net: require of both -external and -ipv6 flags to run IPv6 multicast tests
net: require of both -external and -ipv6 flags to run IPv6 multicast tests

On BSD variants, the tests need surely constructed IPv6
routing table.

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7776043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang