Add WebhookTestCase abstract class for writing webhook tests.

This cuts a ton of code duplication and semi-duplication between the
webhook tests, and thus should make it a lot easier to write new ones.

zerver/fixtures/pagerduty/pagerduty_bad_message_type.json

@@ -0,0 +1,7 @@
+{
+  "messages": [
+    {
+      "type": "incident.triggered"
+    }
+  ]
+}

zerver/fixtures/pagerduty/pagerduty_unknown_message_type.json

@@ -0,0 +1,7 @@
+{
+  "messages": [
+    {
+      "type": "foo"
+    }
+  ]
+}

zerver/lib/test_helpers.py

@@ -359,7 +359,7 @@ class AuthedTestCase(TestCase):
         self.assert_json_success(result)
 
         # Check the correct message was sent
-        msg = Message.objects.filter().order_by('-id')[0]
+        msg = self.get_last_message()
         self.assertEqual(msg.sender.email, email)
         self.assertEqual(get_display_recipient(msg.recipient), stream_name)