httpserver_test: Add ExpectLog to fix CI

The github security advisory feature lets you make private PRs but
it apparently doesn't support CI so this log failure wasn't caught
until after the PR was merged.

tornado/test/httpserver_test.py

@@ -575,9 +575,10 @@ Transfer-Encoding: chunked
                 b"\n", b"\r\n"
             )
         )
-        start_line, headers, response = self.io_loop.run_sync(
-            lambda: read_stream_body(self.stream)
-        )
+        with ExpectLog(gen_log, ".*invalid chunk size", level=logging.INFO):
+            start_line, headers, response = self.io_loop.run_sync(
+                lambda: read_stream_body(self.stream)
+            )
         self.assertEqual(400, start_line.code)
 
     @gen_test