Composer tests now use the multi-company enabled model by default, allowing to
test company-dependent behavior. This has no impact on current tests, as there
is no company-dependent fields on composer model, and all tests are anyway
run into the main company (except multi-company specific tests, suffixed
by '_mc' generally).
We therefore also add some multi-company oriented tests to check notably
return-path or environment companies in various scenarios. Go until the SMTP
generation to test mail server choice, smtp_from and filtering, notifications
email.
Followup of odoo/odoo#136318 and odoo/odoo@3ffa1a0611 notably.
Task-36879 (Mail: Support Multi Domains Aliases)
Part-of: odoo/odoo#76734