split, improve and add agents users CRUD feature specs
Created by: adipasquale
preliminary to #774 : reorganizes agents users CRUD specs for upcoming evolutions
doesn't seem to slow down specs on CircleCI (3m30 for this run)
by chance it also fixes this ticket: https://trello.com/c/NdBzfftm/962-ajouter-un-test-bout-en-bout-sur-la-cr%C3%A9ation-de-proche