test/chan: add missing flag.Parse in doubleselect.go

doubleselect.go defines a flag to control the number of iterations,
but never called flag.Parse so it was unusable.

Change-Id: Ib5d0c7119e7f7c9a808dcc02d0d9cc6ba5bbc16e
Reviewed-on: https://go-review.googlesource.com/41299
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/chan/doubleselect.go

@@ -61,6 +61,7 @@ func recver(in <-chan int) {
 func main() {
 	runtime.GOMAXPROCS(2)
 
+	flag.Parse()
 	c1 := make(chan int)
 	c2 := make(chan int)
 	c3 := make(chan int)