[IMP] mail: clean test about forwarded emails and parent_id

Previous version of the test was actually not correct. Mailgateway was not
considering references as in-reply-to when trying to find answers that have
to be considered as a new thread instead of an answer to an existing thread.

Purpose of the test is actually to be sure that answers considered as a new
thread have no parent_id set. Otherwise you may have threads linked to
messages belonging to other records, leading to access right issues. Moreover
if this email begins a new discussion thread it makes no sense to have a
parent message set.

A new test is written to better simulate the test case. It sets a message_id
containing reply_to which is the way odoo uses to indicated that answers
to a message have to be considered as new threads. This is used notably in
mass mailing for example.