Fix case in wait_for_background_updates where self.store does not exist

tests/unittest.py

@@ -334,7 +334,7 @@ def wait_on_thread(self, deferred, timeout=10):
     def wait_for_background_updates(self) -> None:
         """Block until all background database updates have completed."""
         while not self.get_success(
-            self.store.db_pool.updates.has_completed_background_updates()
+            self.hs.get_datastore().db_pool.updates.has_completed_background_updates()
         ):
             self.get_success(
                 self.store.db_pool.updates.do_next_background_update(100), by=0.1