avoid racey flakes in test_client_storage (#5920)

Author: masenf

tests/integration/test_client_storage.py

@@ -659,8 +659,8 @@ def set_sub_sub(var: str, value: str):
         and isinstance(app_state_manager, StateManagerDisk)
     ):
         # Purge the backend's disk manager
-        del app_state_manager.states[token]
-        del app_state_manager._write_queue[token]
+        app_state_manager.states.pop(token, None)
+        app_state_manager._write_queue.pop(token, None)
         og_token_expiration = app_state_manager.token_expiration
         app_state_manager.token_expiration = 0
         app_state_manager._purge_expired_states()