Purpose of this commit is to add tests for recipients fetch. Purpose is to
assess current behavior before going into some cleaning or refactoring.
Notably when dealing with
* forced recipients (pids, aka 'additional recipients' in composers);
* subtype-based computation (internal, subtype follower);
* multi-users per partner (notably as a result of partner merge);
Future improvements will modify this method. Adding some low level tests
for it will ease development and checking differences.
Some tests are also regrouped in the same test class to lessen noise.
Task-2739294 (Mail: Batch recipients fetch and improve its usage)
Part-of: odoo/odoo#82167
Thibault Delavallée
2ad668d578