test/closure2.go: correctly "use" tmp

cmd/go doesn't complain (this is an open issue), but go/types does

Change-Id: I2caec1f7aec991a9500d2c3504c29e4ab718c138
Reviewed-on: https://go-review.googlesource.com/3541Reviewed-by: Alan Donovan <adonovan@google.com>

test/closure2.go

@@ -54,6 +54,7 @@ func main() {
 	}
 	func() {
 		tmp = w // force capture of w, but do not write to it yet
+		_ = tmp
 		func() {
 			func() {
 				w++ // write in a nested closure