[FIX] mass_mailing: ondelete cascade on transient

The test wizard will be dropped eventually but it is not possible to delete
the mass-mailing before the transient is cleaned too due to the required field.
To make it faster, add a ondelete cascade on the field.

Closes #15217

addons/mass_mailing/wizard/test_mailing.py

@@ -11,7 +11,8 @@ class TestMassMailing(osv.TransientModel):
     _columns = {
         'email_to': fields.char('Recipients', required=True,
             help='Comma-separated list of email addresses.'),
-        'mass_mailing_id': fields.many2one('mail.mass_mailing', 'Mailing', required=True),
+        'mass_mailing_id': fields.many2one(
+            'mail.mass_mailing', 'Mailing', required=True, ondelete='cascade'),
     }
 
     _defaults = {