selftests/landlock: Clean up error logs related to capabilities

It doesn't help to call TH_LOG() for every cap_*() error. Let's only
log errors returned by the kernel, not by libcap specificities.

Link: https://lore.kernel.org/r/20240125153230.3817165-3-mic@digikod.net


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