testscript: fix ptyName() returning /dev/pts/4294967296 on s390x

Use uint32 instead of uint (64-bit in Go on s390x) to store the return
value of the TIOCGPTN syscall.  This is to avoid the 32-bit value from
being stored into a 64-bit buffer and get left-shifted by 32 when
dereferencing, turning what should be /dev/pts/1 to /dev/pts/4294967296
on big-endian architectures such as s390x.

Special thanks to the explanation and a similar bug fix provided at
containerd/console#51

testscript/internal/pty/pty_linux.go

@@ -8,7 +8,7 @@ import (
 )
 
 func ptyName(f *os.File) (string, error) {
-	var out uint
+	var out uint32
 	err := ioctl(f, "TIOCGPTN", syscall.TIOCGPTN, uintptr(unsafe.Pointer(&out)))
 	if err != nil {
 		return "", err