Deprecation warnings

### Problem

When running `mix test` using Cabbage in Elixir 1.18, one gets this warning:

```
    warning: ExUnit.Case.register_test/4 is deprecated. Use register_test/6 instead
    │
  3 │   use Cabbage.Feature, async: false, file: "....feature"
```

I imagine this has been a more recent change in the API and might break after awhile.

Thank you,