Add tests for safety worker

```
crate async_runtime
├── mod safety: pub
│   ├── fn ensure_safety_enabled: pub
│   ├── fn spawn: pub
│   ├── fn spawn_from_boxed: pub
│   ├── fn spawn_from_boxed_on_dedicated: pub
│   ├── fn spawn_from_reusable: pub
│   ├── fn spawn_from_reusable_on_dedicated: pub
│   └── fn spawn_on_dedicated: pub
```