[IMP] test_mail{..mailing/full}: backport tooling improvements

PURPOSE

Purpose of this commit is to backport some improvements done in mail testing
tools done in newer versions. Some of them are required for incoming tests
to be added, other just to try to ease writing tests across versions.

SPECIFICATIONS

Partial backport of odoo/odoo@94208cb8b470bda745f7cbb6bc386f44d7bf14ad

Improve finding outgoing mails and emails when batch methods (mass mailing)
create similar emails, that can be distinguished notably by the subject
in addition to from / to.

Partial backport of odoo/odoo@c98f259736b5aa9c37a3580b2dad5795e8d5f6df

Add a method to check MailMail, based on a given record. When having duplicates
to differentiate in a given mailing, having just recipients is not sufficient
as multiple emails may match a given recipients list. Checking model / res_id
is another method for finding emails.

Partial backport of odoo/odoo@a3e404e17ea36447e0bbe5f92aef94e26c299366

Add some information and values propagation in some custom asserts in mail
test tooling.

Partial backport of odoo/odoo@b915617569c1de1ddef8c116aa1f7c9f66037598

Allow to return <mail.mail> records and found outgoing emails when using
asserts. It eases doing checks in some specific tests e.g. checking
notification layout usage in emails. Indeed this is quite low level and
does not really deserves its own assert tooling methods.

Partial backport of odoo/odoo@bbf4783ac68f296dd028b46aff882b4187a0f9fb

Improve checks and asserts done when asserting content of produced
mailing content (mails, traces, ...).

Task-3438381 (TestMail: backport tools)
Prepares Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#129508