[FW][FIX] mass_mailing_sms: try to render jinja content when using test wizards

# Purpose

The wizard to test a mailing does not compute the jinja part. It does
not detect syntax errors that would make cron crash

# Specifications

If there is at least one record in the mailing model then render template
completely. Thow an error if rendering cannot be done.

The downside is that we cannot test the jinja part if there is not yet any
record in the mailing's model.

Do it for mass_mailing and mass_mailing_sms.

Add tests to ensure error detection in test wizard effectively works.

Task ID-2312442
PR #55696

closes odoo/odoo#60370

Forward-port-of: odoo/odoo#55696
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>