Purpose of this commit is to add some first tests about rating and rating
mixin. Indeed those are not really tested by standard python unit tests.
In this commit we add tests about
* rating preparation: token, information from the record to rate;
* rating apply: posting message, author;
Task-2673913 (TestMail: Cleanup and improve test coverage)
Task-2704390 (TestMailFull: add first rating tests)
Part-of: odoo/odoo#86393
