[FIX] auth_signup: fix email sent when testing an import

Issue

    - Contacts
    - Import a contact that will be a portal user
    - Test import

    Activation email sent

Cause

    Testing an import do the whole process including sending an email
    because of force_send (email not rollbacked).

Solution

    Check if we are testing the import. If yes, do not force
    send the email. So, the email will be rollbacked.

OPW-2168868

closes odoo/odoo#43724

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>