net: fix leak in test
net: fix leak in test

Also change the Listner variable name from l to ln.

R=golang-dev, rsc, dave
CC=golang-dev
http://codereview.appspot.com/5918046

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