Character::i_add returns an item_location

[mqrause]

Summary

Infrastructure "Character::i_add returns an item_location"

Purpose of change

Initiated as a fix for the errors that popped up in #58093
Handling this with item_location is also generally preferable to item pointers or references.

Describe the solution

• adjust Character::i_add and everything around it
• adjust everything in tests
• fix a bug in Character::best_pocket that popped up with this (parent of returned item_location is ::item_in_container when it should be ::item_on_person

While I was there I also did some minor adjustments in the tests, but only in lines I had to change anyway or lines next to them. Mostly stuff like changing CHECK( x == true ) to CHECK( x ).

Describe alternatives you've considered

Maybe also have map::add_item return an item_location, but this turned out bigger than expected already, so maybe another time.

Testing

Picking stuff up and spawning it with debug menu seems to work properly. Had no test issues either.
Making an ammo belt from linkage on the ground works, although that always only loads one bullet. Probably linkage on the ground isn't properly registered as available for reloading, but that's a different bug in a different place and doesn't break anything either.

Additional context

Going through the tests showed me there are tons of methods that should also should be changed to accept item_locations as parameters instead of item * or item &. Basically everything that handles real, existing items. Wield, wear, reload, that kind of stuff. But that's not a project I'm going to tackle any time soon.