selftests/seccomp: Add test for atomic addfd+send

This just adds a test to verify that when using the new introduced flag
to ADDFD, a valid fd is added and returned as the syscall result.

Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210517193908.3113-5-sargun@sargun.me