[IMP] mail, *: always mount discuss/chatWindow/dialog during tests

*: hr, im_livechat, snailmail, website_livechat.

This PR prepares the ground for  the one introducing the new environment in the
discuss app. Indeed, tests will now use the mainComponentRegistry in order to mount
those components. Doing so, the parameters has{Discuss/Dialog/ChatWindow} will become
obsolete. Removing those parameters now helps reducing the noise in the main PR.

task-2582313

closes odoo/odoo#90120

Related: odoo/enterprise#26759
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>