increase the max request size for test worker HSes

... because it turns out some replication requests are bigger than 1234 bytes.

tests/replication/_base.py

@@ -359,7 +359,7 @@ def make_worker_hs(
             config=worker_hs.config.server.listeners[0],
             resource=resource,
             server_version_string="1",
-            max_request_body_size=1234,
+            max_request_body_size=4096,
             reactor=self.reactor,
         )