[IMP] crm: add test for '_message_get_suggested_recipients'

This method is still not really tested, except its override of 'mail.thread.cc'.
So better have a test, especially in CRM that has some specific overrides.
Formatted email and multi-email input are tested, to check their current
support. Next commits will try to improve that, notably avoid formatting
issues.

Task-2612945 (Mail: Defensive email formatting)

X-original-commit: odoo/odoo@340c362798b1cdb2ddb171de385bca45677724af
Part-of: odoo/odoo#133958