[REF] test_mail: update test mail gateway 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 tests to new tools / asserts / helpers / classes defined in mail
and test_mail. 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);

In this commit we update: test mail gateway

LINKS

Task ID 2068986
PR #38070

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