[IMP] base: insert dummy SMTP server

The current neutralization implementation for ir_mail_server deactivates
existing servers. This can still result in mails being sent if an SMTP
server was specified on the command-line. To prevent that from happening
a dummy SMTP server is defined that doesn't resolve to anything.

I've opted to use the "invalid" domain for this purpose:
https://www.rfc-editor.org/rfc/rfc6761#section-6.4

 .

closes odoo/odoo#130376

Signed-off-by: Merel Geens <mege@odoo.com>