webhook tests: Rename main helper to check_webhook.

Almost all webhook tests use this helper, except a few
webhooks that write to private streams.

Being concise is important here, and the name
`self.send_and_test_stream_message` always confused
me, since it sounds you're sending a stream message,
and it leaves out the webhook piece.

We should consider renaming `send_and_test_private_message`
to something like `check_webhook_private`, but I couldn't
decide on a great name, and it's very rarely used.  So
for now I just made sure the docstrings of the two
sibling functions reference each other.

zerver/webhooks/gocd/tests.py

@@ -14,8 +14,8 @@ class GocdHookTests(WebhookTestCase):
                             "/go/tab/pipeline/history/pipelineName)\n"
                             "Comment: my hola mundo changes")
 
-        self.send_and_test_stream_message(
-            'pipeline',
+        self.check_webhook(
+            "pipeline",
             self.TOPIC,
             expected_message,
             content_type="application/x-www-form-urlencoded",
@@ -28,8 +28,8 @@ class GocdHookTests(WebhookTestCase):
                             "/go/tab/pipeline/history/pipelineName)\n"
                             "Comment: my hola mundo changes")
 
-        self.send_and_test_stream_message(
-            'pipeline_failed',
+        self.check_webhook(
+            "pipeline_failed",
             self.TOPIC,
             expected_message,
             content_type="application/x-www-form-urlencoded",