net: disable unixgram test on Plan 9 and Windows
net: disable unixgram test on Plan 9 and Windows

Fixes build on Plan 9 and Windows.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7454049

wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old