Fix require torchaudio and refactor test requirements
Currently there is a bug in `require_torchaudio` (indeed it is requiring `sox` instead):
```python
def require_torchaudio(test_case):
    if find_spec("sox") is None:
...
```

The bug was introduced by:
- #3685
  - Commit: https://github.com/huggingface/datasets/pull/3685/commits/b5a3e7122d49c4dcc9333b1d8d18a833fc04b940

which moved
```python
require_sndfile = pytest.mark.skipif(
    # In Windows and OS X, soundfile installs sndfile
    (sys.platform != "linux" and find_spec("soundfile") is None)
    # In Linux, soundfile throws RuntimeError if sndfile not installed with distribution package manager
    or (sys.platform == "linux" and find_library("sndfile") is None),
    reason="Test requires 'sndfile': `pip install soundfile`; "
    "Linux requires sndfile installed with distribution package manager, e.g.: `sudo apt-get install libsndfile1`",
)
require_sox = pytest.mark.skipif(
    find_library("sox") is None,
    reason="Test requires 'sox'; only available in non-Windows, e.g.: `sudo apt-get install sox`",
)
require_torchaudio = pytest.mark.skipif(find_spec("torchaudio") is None, reason="Test requires 'torchaudio'")
```
to
```python
def require_sndfile(test_case):
    """
    Decorator marking a test that requires soundfile.
    These tests are skipped when soundfile isn't installed.
    """
    if (sys.platform != "linux" and find_spec("soundfile") is None) or (
        sys.platform == "linux" and find_library("sndfile") is None
    ):
        test_case = unittest.skip(
            "test requires 'sndfile': `pip install soundfile`; "
            "Linux requires sndfile installed with distribution package manager, e.g.: `sudo apt-get install libsndfile1`",
        )(test_case)
    return test_case


def require_sox(test_case):
    """
    Decorator marking a test that requires sox.
    These tests are skipped when sox isn't installed.
    """
    if find_library("sox") is None:
        return unittest.skip("test requires 'sox'; only available in non-Windows, e.g.: `sudo apt-get install sox`")(
            test_case
        )
    return test_case


def require_torchaudio(test_case):
    """
    Decorator marking a test that requires torchaudio.
    These tests are skipped when torchaudio isn't installed.
    """
    if find_spec("sox") is None:
        return unittest.skip("test requires 'torchaudio'")(test_case)
    return test_case
```

This PR;
- fixes the bug in `require_torchaudio`
- refactors the test requirements back to using `pytest` instead of `unittest`
  - the text in `pytest.skipif` `reason` can be used if needed in a test body: `require_torchaudio.kwargs["reason"]`