[ADD] website_sale_coupon : allows to give a coupon to customer, with validity, available for some products

- Task: https://www.odoo.com/web#id=9507&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.ShuFMqJRjlBKDtpo
Put a common file holding test data creation in the setup. It will reduce the size of the test itself (separate data from tests). Moreover you will be able to write several small tests without havign to repeat the data creation process each time.