selftests/landlock: Add cred_transfer test

Check that keyctl(KEYCTL_SESSION_TO_PARENT) preserves the parent's
restrictions.

Fixes: e1199815 ("selftests/landlock: Add user space tests")
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20240724.Ood5aige9she@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>