test: test that x := <-c accepts a general expression

The gccgo compiler used to fail to parse this.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5610051

test/chan/select3.go

@@ -197,13 +197,13 @@ func main() {
 	})
 	testBlock(never, func() {
 		select {
-		case x := <-closedch:
+		case x := (<-closedch):
 			_ = x
 		}
 	})
 	testBlock(never, func() {
 		select {
-		case x, ok := <-closedch:
+		case x, ok := (<-closedch):
 			_, _ = x, ok
 		}
 	})