Bug 34008: Harmonize attribute names

Bug 34008: Harmonize attribute names

This patch harmonizes the attribute names with what is used for `items`
and `checkouts` in terms of terminology.

It also adapts the tests so they are less random failure-prone (they had
a fixed value for the item type, which might make things explode if the
chosen value already exists on the DB.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 78589dbe6ecf1a4126b0420894a732bd58a9752a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>