[FIX] link_tracker: stop performing external HTTP calls

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#128265

X-original-commit: e4c16770
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>