[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
`request.env['res.users'].send_forum_validation_email(forum_id=forum_id)` does not work?