net: make sure to use /tmp in unix domain socket tests
net: make sure to use /tmp in unix domain socket tests

R=golang-dev, bradfitz, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7547044

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