APNs: Fix a case I broke while working out tests.

I got distracted, came back later to a successful test run in my
terminal, and thought I remembered finishing the change and just
kicking off a final test run to check.

In fact, there was an `assert False` right in the normal case for
production, and I just hadn't finished a test for that path. (m.-)
Definitely the most grateful I've been for our coverage checks,
which highlighted this for me.

Remove the `assert False`, and also finish writing the test it was
there to help me write.  Those lines are covered now.

zerver/lib/push_notifications.py

@@ -66,7 +66,6 @@ def get_apns_client() -> APNsClient:
         if settings.APNS_CERT_FILE is not None:
             _apns_client = APNsClient(credentials=settings.APNS_CERT_FILE,
                                       use_sandbox=settings.APNS_SANDBOX)
-            assert False
         _apns_client_initialized = True
     return _apns_client
 

zerver/tests/test_push_notifications.py

@@ -565,6 +565,12 @@ class TestAPNs(PushNotificationTest):
         apn.send_apple_push_notification(
             self.user_profile.id, devices, payload_data)
 
+    def test_get_apns_client(self) -> None:
+        with self.settings(APNS_CERT_FILE='/foo.pem'), \
+                mock.patch('zerver.lib.push_notifications.APNsClient') as mock_client:
+            client = get_apns_client()
+            self.assertEqual(mock_client.return_value, client)
+
     def test_not_configured(self) -> None:
         with mock.patch('zerver.lib.push_notifications.get_apns_client') as mock_get, \
                 mock.patch('zerver.lib.push_notifications.logging') as mock_logging: