fix flaky test `test_timeout_stops_execution_in_sync_subflows`

### Bug summary

https://github.com/PrefectHQ/prefect/actions/runs/12206604955/attempts/1?pr=16255


```
pytest -n5 tests/test_flows.py -k test_timeout_stops_execution_in_sync_subflows --flake-finder
```

### Version info

```Text
main
```


### Additional context

_No response_