selftests/seccomp: Ensure that notifications come in FIFO order

When multiple notifications are waiting, ensure they show up in order, as
defined by the (predictable) seccomp notification ID. This ensures FIFO
ordering of notification delivery as notification ids are monitonic and
decided when the notification is generated (as opposed to received).

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: linux-kselftest@vger.kernel.org
Acked-by: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220428015447.13661-2-sargun@sargun.me