[FIX] portal: fix regression of API migration in the portal wizard

Before the migration, send_mail() used to be called with an untainted
context but after the API migration it inherited the altered context
with active_test=False which caused subsequent calls to
ir.mail_server#connect() to fetch inactive records.

addons/portal/wizard/portal_wizard.py

@@ -149,7 +149,7 @@ class PortalWizardUser(models.TransientModel):
                     wizard_user.user_id.write({'active': True, 'groups_id': [(4, group_portal.id)]})
                     # prepare for the signup process
                     wizard_user.user_id.partner_id.signup_prepare()
-                    wizard_user._send_email()
+                    wizard_user.with_context(active_test=True)._send_email()
                 wizard_user.refresh()
             else:
                 # remove the user (if it exists) from the portal group