[FIX] mass_mailing: preserve comments when testing a mailing

When sending a mailing we make sure to preserve comments (in particular
so that MSO comments can be read by Outlook). However this was not the
case when testing a mailing using the Test button in the form view.

task-3488162
opw-3290548
opw-3479234

closes odoo/odoo#133754

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/wizard/mailing_mailing_test.py

@@ -39,7 +39,7 @@ class TestMassMailing(models.TransientModel):
         # Downside: Qweb syntax is only tested when there is atleast one record of the mailing's model
         if record:
             # Returns a proper error if there is a syntax error with Qweb
-            body = mailing._render_field('body_html', record.ids, post_process=True)[record.id]
+            body = mailing.with_context(preserve_comments=True)._render_field('body_html', record.ids, post_process=True)[record.id]
             preview = mailing._render_field('preview', record.ids, post_process=True)[record.id]
             full_body = mailing._prepend_preview(Markup(body), preview)
             subject = mailing._render_field('subject', record.ids)[record.id]