[IMP] link_tracker, (test_)mass_mailing: improve test tools and clean existing tests

URPOSE

Prepare code cleaning and optimization in mail, mass_mailing and SMS by
cleaning models for readability and code complexity and footprint reduction.

SPECIFICATIONS

Add tool methods and asserts for link tracker tests.

Add tool methods and assets for mass mailing tests, notably about mailing
traces and their link with emails.

Merge existing and duplicated tests and rewrite them a bit.

Rename and improve test_mass_mailing models.

LINKS

Prepares task ID 2238597 (notification and trace models cleaning)
Task ID 1906925 (mass mailing tests cleaning)
PR #50169

Related: odoo/enterprise#10180
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>