syscall: use libc-based ioctl for Ioctl in tests on darwin
    
    Direct syscalls are no longer supported on darwin, instead wrap the
    existing func ioctl for tests.
    
    Change-Id: Ie2c5b6e5f54e992f4d6b21513ca8f89fcf28ef10
    Reviewed-on: https://go-review.googlesource.com/c/go/+/256219
    Trust: Tobias Klauser <tobias.klauser@gmail.com>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>