[IMP] mail: tests: clean server actions tests
[IMP] mail: tests: clean server actions tests

Purpose is to lessen noise and make tests less low-level. Including :

 * remove unnecessary fail string;
 * remove unnecessary method return value test as this is low level;

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo