PURPOSE
Improve tooling of mail tests. Lessen custom code, try to use same tools
in all tests.
SPECIFICATIONS
Use MailCommon as a base class for performance test. It now uses the same
boilerplate as other tests, leading to easier to reproduce and understand
testing setup.
This notably now adds outgoing mail servers which means adding one query
to some tests involving emails.
Task-2673913 (TestMail: Cleanup and improve test coverage)
Part-of: odoo/odoo#86393
Thibault Delavallée
e31504f3a3