selftest/seccomp: add a new test for the sync mode of seccomp_user_notify

Test output:
 #  RUN           global.user_notification_sync ...
 #            OK  global.user_notification_sync
 ok 51 global.user_notification_sync

Signed-off-by: Andrei Vagin <avagin@google.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230308073201.3102738-6-avagin@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>