[ADD,IMP]Website_forum:Add a python test case

- Task: https://www.odoo.com/web#id=8745&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.EsLOcZQQuvZYB4ll
https://github.com/odoo/odoo/blob/8.0/openerp/api.py#L741