Extend main test for service usage page

This adds missing assertions for email and SMS usage, as well as
letters with the help of some additional test data.

Previously we were only checking monthly usage (in other tests).

tests/conftest.py

@@ -2350,7 +2350,14 @@ def mock_get_usage(mocker, service_one, fake_uuid):
                 "notifications_sent": 90,
                 "rate": 0.0165,
                 "letter_total": 0
-            }
+            },
+            {
+                "notification_type": "letter",
+                "chargeable_units": 300,
+                "notifications_sent": 100,
+                "rate": 0.1,
+                "letter_total": 30
+            },
         ]
 
     return mocker.patch(