[REF] test_mass_mailing: update module to new common

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

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