Fix tests for 3.16

These patches fixes testing against Koha 3.16, mainly doing three things:
- small bug in db_dependent tests on $item
- Catalogue.t : mock database dependent calls (items_columns & item available) in Catalogue.pm (should not be db dependent)
- test @items response in CanItemBeReserved so it doesnt complain if no array response
This is useless stuff that should be removed