[FIX] link_tracker: stop performing external HTTP calls during tests

Only tests tagged as external should actually call external sites. `TestMailRenderMixin` not only does that but it calls URLs which literally don't exist either.

Patch `requests.Session.request` (via a utility added to odoo tests common) to disable that in the class.

closes odoo/odoo#128249

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>