Test can get letter template

It works already, just making sure it stays working.

tests/app/conftest.py

@@ -180,7 +180,7 @@ def sample_template(notify_db,
         'created_by': created_by,
         'archived': archived
     }
-    if template_type == 'email':
+    if template_type in ['email', 'letter']:
         data.update({
             'subject': subject_line
         })

tests/app/template/test_rest.py

@@ -321,6 +321,11 @@ def test_should_get_only_templates_for_that_service(notify_api, sample_user, ser
             None,
             'hello ((name)) we’ve received your ((thing))',
             'sms'
+        ),
+        (
+            'about your ((thing))',
+            'hello ((name)) we’ve received your ((thing))',
+            'letter'
         )
     ]
 )