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/main/views/notifications.py

@@ -25,6 +25,7 @@ from app import (
     notification_api_client,
 )
 from app.main import main
+from app.notify_client.api_key_api_client import KEY_TYPE_TEST
 from app.template_previews import get_page_count_for_letter
 from app.utils import (
     DELIVERED_STATUSES,
@@ -144,7 +145,10 @@ def get_single_notification_partials(notification):
         ),
         'status': render_template(
             'partials/notifications/status.html',
-            notification=notification
+            notification=notification,
+            sent_with_test_key=(
+                notification.get('key_type') == KEY_TYPE_TEST
+            ),
         ),
     }