[REF] test_mass_mailing: make performance test class child of TransactionCase
[REF] test_mass_mailing: make performance test class child of TransactionCase

It eases manipulation and lessen overhead. Performance tests should be self
contained as much as possible.

Commit linked to task ID 1902307.
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo