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.

tests/conftest.py

@@ -2640,7 +2640,8 @@ def mock_get_notification(
     redact_personalisation=False,
     template_type=None,
     template_name='sample template',
-    is_precompiled_letter=False
+    is_precompiled_letter=False,
+    key_type=None,
 ):
     def _get_notification(
         service_id,
@@ -2670,6 +2671,8 @@ def mock_get_notification(
             is_precompiled_letter=is_precompiled_letter,
             name=template_name
         )
+        if key_type:
+            noti['key_type'] = key_type
         return noti
 
     return mocker.patch(