[IMP] test_mail: add a complex performance test for message_post
[IMP] test_mail: add a complex performance test for message_post

It includes notification on inbox / email as well as attachments management.
This latter one adds quite a lot of queries and is not currently tested
for performances. Future commits will improve those counters.

Related to task 1943901
Linked to PR #32404
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo