Add a test for sending email to provider

We had a test like this for sending sms, but not email. This meant that,
for example, we weren’t checking that the provider was getting passed
the HTML and plain text versions of the email.

tests/app/notifications/rest/test_send_notification.py

@@ -125,7 +125,7 @@ def test_send_notification_with_placeholders_replaced(notify_api, sample_email_t
             )
             assert response.status_code == 201
             assert encryption.decrypt(app.celery.tasks.send_email.apply_async.call_args[0][0][2]) == data
-            assert response_data['body'] == 'Hello Jo'
+            assert response_data['body'] == 'Hello Jo\nThis is an email from GOV.UK'
             assert response_data['subject'] == 'Jo'