selftests/landlock: Add tests for execveat + AT_EXECVE_CHECK

selftests/landlock: Add tests for execveat + AT_EXECVE_CHECK

Extend layout1.execute with the new AT_EXECVE_CHECK flag.  The semantic
with AT_EXECVE_CHECK is the same as with a simple execve(2),
LANDLOCK_ACCESS_FS_EXECUTE is enforced the same way.

Cc: Günther Noack <gnoack@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-5-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>