net: disable "udp" to IPv6 unicast address loopback test on dragonfly
net: disable "udp" to IPv6 unicast address loopback test on dragonfly

Disable the "udp" to IPv6 unicast address on the loopback interface
test under DragonFly BSD. This currently returns a local address of
0.0.0.1, rather than an IPv6 address with zone identifier.

Update issue 7473

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://codereview.appspot.com/71500044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang