Fix sync join/ban race test failing

We're now better at rejecting this

tests/handlers/test_sync.py

@@ -19,6 +19,7 @@
 #
 from typing import Collection, ContextManager, List, Optional
 from unittest.mock import AsyncMock, Mock, patch
+from http import HTTPStatus
 
 from parameterized import parameterized, parameterized_class
 
@@ -347,7 +348,15 @@ def test_ban_wins_race_with_join(self) -> None:
         # the prev_events used when creating the join event, such that the ban does not
         # precede the join.
         with self._patch_get_latest_events([last_room_creation_event_id]):
-            self.helper.join(room_id, eve, tok=eve_token)
+            self.helper.join(
+                room_id,
+                eve,
+                tok=eve_token,
+                # Previously, this join would succeed but now we expect it to fail at
+                # this point. The rest of the test is for the case when this used to
+                # succeed.
+                expect_code=HTTPStatus.FORBIDDEN,
+            )
 
         # Eve makes a second, incremental sync.
         eve_incremental_sync_after_join: SyncResult = self.get_success(