PURPOSE
Currently tools and asserts for mail tests are located inside test_mail
module. It makes difficult to re-use them in apps tests or force them to
write custom quick and dirty tools and asserts.
SPECIFICATIONS
Update this module to new tools / asserts / helpers / classes defined in mail
and test_mail.
In this commit we remove the ``MassMailingCase`` and replace it by a fresh
``TestMailCommon`` that is simply updated to hold a marketing user by default.
That way all mocks and asserts are available in all sub test classes.
Coming from common update, we have notably
* ``BaseFunctionalTest`` class is replaced by the new ``TestMailCommon``
pimped one;
* ``assertNotifications`` is replaced by ``assertSinglePostNotifications``
(shortcut for a simple message_post) or ``assertPostNotifications``
(complete asserts involving several messages and notifications);
* correctly invoke ``mock_mail_gateway`` when mocking email sending;
* replace manual check of sent emails or ``assertEmails`` by
``assertSentEmails``;
* remove custom mocks / checks and replace them by now standard mocks
and assertions (if any);
LINKS
Task ID 2068986
PR #38070
Thibault Delavallée
b287504977