[IMP] (test_)mail: improve some testing tools and helpers for notifications

Cleanup some code bits in common classes, add some docstrings. Improve
notifications related helpers, notably to ease checking content of mail.mail
or outgoing emails when posting messages.

Update 'test_message_post' with those new helpers, to ease inclusion of
additional specific values test with alias domains in mind in next commits.

Prepares Task-36879 (Mail: Support MultiCompany Aliases)

Part-of: odoo/odoo#136102