Purpose of this commit is to add tests on ``failure_reason`` usage of notification model. Tests already exists for MailMail and other fields but not that specific one. Followup of odoo/odoo@d895f3514e New tests are added to check the usage of email_to and email_cc when sending emails. Notably wrong usage of email_cc (lost formatting, copy of all sent emails) is asserted to be fixed afterwards. A performance test about batch sending is also added. Prepares Task-2684479 (Mail: Better send error storage and display) Part-of: odoo/odoo#96223