os/exec: fix test hang

Currently the test closes random files descriptors,
which leads to hang (in particular if netpoll fd is closed).
Try to open only fd 3, since the parent process expects it to be fd 3 anyway.
Fixes #5571.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9778048

src/pkg/os/exec/exec_test.go

@@ -540,13 +540,11 @@ func TestHelperProcess(*testing.T) {
 		n, _ := strconv.Atoi(args[0])
 		os.Exit(n)
 	case "describefiles":
-		for fd := uintptr(3); fd < 25; fd++ {
-			f := os.NewFile(fd, fmt.Sprintf("fd-%d", fd))
-			ln, err := net.FileListener(f)
-			if err == nil {
-				fmt.Printf("fd%d: listener %s\n", fd, ln.Addr())
-				ln.Close()
-			}
+		f := os.NewFile(3, fmt.Sprintf("fd3"))
+		ln, err := net.FileListener(f)
+		if err == nil {
+			fmt.Printf("fd3: listener %s\n", ln.Addr())
+			ln.Close()
 		}
 		os.Exit(0)
 	case "extraFilesAndPipes":