[IMP] mail: tests: clean and improve mail followers tests
[IMP] mail: tests: clean and improve mail followers tests

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

 * add test for the various auto subscription cases, notably through
   umbrella (like project -> task auto subscription);
 * remove low-level tests; there is no need to test the output of add
   followers method as it is only an implementation details;

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