timit_asr doesn't load the test split.
The [timit_asr.py](https://huggingface.co/datasets/timit_asr/blob/main/timit_asr.py) script iterates over the WAV files per split directory using this:
```python
wav_paths = sorted(Path(data_dir).glob(f"**/{split}/**/*.wav"))
wav_paths = wav_paths if wav_paths else sorted(Path(data_dir).glob(f"**/{split.upper()}/**/*.WAV"))
```

Can you check that there is a directory named "test" somewhere in your timit data directory ?
### Describe the bug

When I use the function ```timit = load_dataset('timit_asr', data_dir=data_dir)```, it only loads train split, not test split.

I tried to change the directory and filename to lower case to upper case for the test split, but it does not work at all.

```python
DatasetDict({
    train: Dataset({
        features: ['file', 'audio', 'text', 'phonetic_detail', 'word_detail', 'dialect_region', 'sentence_type', 'speaker_id', 'id'],
        num_rows: 4620
    })
    test: Dataset({
        features: ['file', 'audio', 'text', 'phonetic_detail', 'word_detail', 'dialect_region', 'sentence_type', 'speaker_id', 'id'],
        num_rows: 0
    })
})
```

The directory structure of both splits are same. (DIALECT_REGION / SPEAKER_CODE / DATA_FILES)

### Steps to reproduce the bug

1. just use ```timit = load_dataset('timit_asr', data_dir=data_dir)```

### Expected behavior

```python
DatasetDict({
    train: Dataset({
        features: ['file', 'audio', 'text', 'phonetic_detail', 'word_detail', 'dialect_region', 'sentence_type', 'speaker_id', 'id'],
        num_rows: 4620
    })
    test: Dataset({
        features: ['file', 'audio', 'text', 'phonetic_detail', 'word_detail', 'dialect_region', 'sentence_type', 'speaker_id', 'id'],
        num_rows: 1680
    })
})
```

### Environment info

- ubuntu 20.04
- python 3.9.13
- datasets 2.7.1