Split is inferred from filename and overrides metadata.jsonl
## Describe the bug

Including the strings "test" or "train" anywhere in a filename causes `datasets` to infer the split and silently ignore all other files.

This behavior is documented for directory names but not filenames: https://huggingface.co/docs/datasets/image_dataset#imagefolder

## Steps to reproduce the bug
`metadata.jsonl`
```json
{"file_name": "photo of a cat.jpg", "text": "a photo of a cat"}
{"file_name": "photo of a dog.jpg", "text": "a photo of a dog"}
{"file_name": "photo of a train.jpg", "text": "a photo of a train"}
{"file_name": "photo of test tubes.jpg", "text": "a photo of test tubes"}
```

`bug.py`
```python
from datasets import load_dataset

dataset = load_dataset("dataset")

print(dataset)
# DatasetDict({
#     train: Dataset({
#         features: ['image', 'text'],
#         num_rows: 1
#     })
#     test: Dataset({
#         features: ['image', 'text'],
#         num_rows: 1
#     })
# })

for split in dataset:
    for n in dataset[split]:
        print(n['text'])
        # a photo of a train
        # a photo of test tubes
```

## Expected results
One single dataset with all four images / a warning for unused files / documentation of this behavior

## Actual results
Only the images with "test" or "train" in the name are loaded

## Environment info
- `datasets` version: 2.5.1
- Platform: macOS-12.5.1-x86_64-i386-64bit
- Python version: 3.10.4
- PyArrow version: 9.0.0
- Pandas version: 1.5.0