Fix TestCommand to move dataset_infos instead of copying
All the datasets that are loaded normally with `load_dataset`, if `dataset_infos.json` exists, have this file in the importable directory. So it's fine if we copy the file instead of moving it but it's not a big deal.

Any reason to prefer moving it rather than copying it ? @lvwerra reported than when generating the `dataset_infos.json` for multiple dataset directories containing only JSONL files, subsequent `dataset_infos.json` files contained all previous directories as configs:
- First generate metadata for dataset in dir `dir1`: dataset_infos.json contains one config for `dir1`
- Then generate metadata for dataset in dir `dir2`: dataset_infos.json contains 2 configs, for `dir1` and `dir2`

The reason is that all dataset_infos.json files are first created in the same dir (the one containing the json builder) and then **copied** to the user dir.

Subsequent calls of TestCommand don't replace the dataset_infos.json already present in the dir of the json builder, but append to it.

MAYBE: we should just move for this use case, and copy for the other use cases? See this use case here:
- #3680 @lhoestq aren't you mentioning the case in the else clause?
```python
else:  # in case of a remote dataset
    dataset_dir = None
```

In that case `dataset_infos.json` is not copied: `dataset_dir = None` When using the JSON loader, calling `get_imported_module_dir()` returns a path inside the pip installed packages, so we shouldn't write files in it anyway, and the dataset_infos.json file should be written directly in the user's directory instead (some users don't have write access to the pip installed packages for example).

Maybe the packaged modules like `json` should override `_save_infos` to save them in the user's directory instead of next to the builder's script. What do you think ? Anyway as a hotfix we can just add an exception for the `json` builder for now, if the issue has to be fixed soon I'm closing this PR.
Why do we copy instead of moving the file?

CC: @lhoestq @lvwerra 