selftests/seccomp: Check ENOSYS under tracing

There should be no difference between -1 and other negative syscalls
while tracing.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Will Deacon <will@kernel.org>
Cc: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>