selftests/seccomp: Test thread vs process killing

This verifies that SECCOMP_RET_KILL_PROCESS is higher priority than
SECCOMP_RET_KILL_THREAD. (This also moves a bunch of defines up earlier
in the file to use them earlier.)

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>