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

gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,gnanderson/go,dsriram/golang