Indicate messages sent with a test key

If a message has been sent with a test key it’s a bit confusing to just
say ‘Delivered’ on the page, because it hasn’t gone to anyone’s phone.

So this commit adds a bit of hint text to disambiguate what ‘Delivered’
actually means in this context.

app/templates/partials/notifications/status.html

@@ -9,5 +9,8 @@
     {% if notification.status|format_notification_status_as_url %}
       </a>
     {% endif %}
+    {% if sent_with_test_key %}
+      (test)
+    {% endif %}
   </p>
 </div>