Fix ptrace_test:ptrace__syscall_args after ATF upgrade

ATF now opens the results file (without CLOEXEC), so the child actually
has a valid file descriptor 3. To fix this simply use a large number that
will definitely not be a valid file descriptor.

Reviewed by: jhb, cem, lwhsu
Differential Revision: https://reviews.freebsd.org/D28889

(cherry picked from commit 17cc20092cd7a45adb6d772e0f449617007a82d9)