exp/winfsnotify: fix data race in TestNotifyClose
exp/winfsnotify: fix data race in TestNotifyClose

Fixes issue 4342.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6850080

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