Purpose of this commit is to add tests for exclusion list and duplicates
management in composer.
When being in email mode (mass_mail) state of emails is pre-processed in
order to already flag emails that should not be sent or will bounce back.
Notably emails in exclusion list or duplicates emails are set as cancel
with the right failure type.
Some tests already exist at higher level in mass mailing but having tests
at composer level is better when trying to improve code and add tests for
corner cases.
Task-3132710 (Mail: Configurable composer)
Part-of: odoo/odoo#99482
Thibault Delavallée
1394903873