Bug 22284: (QA follow-up) Make pickup locations be Koha::Library objects

Bug 22284: (QA follow-up) Make pickup locations be Koha::Library objects

This patch makes the following methods return array references of
Koha::Library objects instead or unblessed objects;

- Koha::Item->pickup_locations
- Koha::Biblio->pickup_locations
- Koha::Libraries->pickup_locations

Bonus:

- The template plugin is adjusted to unbless things to keep behavior
- Tests are moved to the right .t file.
- Tests for the new behavior are added.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>