Tests fail with Click 8.2 due to removed mix_stderr argument

`tests/test_krr.py` and `tests/test_runner.py` create `CliRunner(mix_stderr=False)`.

Click 8.2 removed the `mix_stderr` parameter, so pytest fails during collection with:

```text
TypeError: CliRunner.__init__() got an unexpected keyword argument 'mix_stderr'
```

Replacing those calls with `CliRunner()` restores test collection with current Click.
