Purpose of this commit is to use the newly introduced helper to create
test users in a quick way and reduce code duplication.
This commit is linked to task ID 1889703 and PR #27526.
Purpose of this commit is to move tests not completely specific to some
mass mailing models to test_mass_mailing. Purpose is to allow to write
more in-depth tests based on existing ones by using test models instead
of some standard odoo ones.
This commit is linked to task ID 1889703 and PR #27526.