Tie the example for AsyncHTTPTestCase to real code.

Now all the reader has to do is copy one of the "Hello, world"
examples to hello.py, copy the test example to test_hello.py, import
AsyncHTTPTestCase, and run the test. It's still not entirely seamless
and idiot-proof, but it's progress!

Of note: the example test can now actually assert things about the
response it receives.

tornado/testing.py

@@ -331,14 +331,19 @@ class AsyncHTTPTestCase(AsyncTestCase):
     Tests will typically use the provided ``self.http_client`` to fetch
     URLs from this server.
 
-    Example::
+    Example, assuming the "Hello, world" example from the user guide is in
+    ``hello.py``::
 
-        class MyHTTPTest(AsyncHTTPTestCase):
+        import hello
+
+        class TestHelloApp(AsyncHTTPTestCase):
             def get_app(self):
-                return Application([('/', MyHandler)...])
+                return hello.make_app()
 
             def test_homepage(self):
                 response = self.fetch('/')
+                self.assertEqual(response.code, 200)
+                self.assertEqual(response.body, 'Hello, world')
 
     That call to ``self.fetch()`` is equivalent to ::