selftests/seccomp: Add test for wait killable notifier

This verifies that if a filter is set up with the wait killable feature
that it obeys the semantics that non-fatal signals are ignored during
a notification after the notification is received.

Cases tested:
 * Non-fatal signal prior to receive
 * Non-fatal signal during receive
 * Fatal signal after receive

The normal signal handling is tested in user_notification_signal. That
behaviour remains unchanged.

On an unsupported kernel, these tests will immediately bail as it relies
on a new seccomp flag.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503080958.20220-4-sargun@sargun.me