[Tests] Test files locally

[patrickvonplaten]

This PR adds a aws and a local decorator to the tests so that tests now run on the local datasets.

By default, the aws is deactivated and local is activated and slow is deactivated, so that only 1 test per dataset runs on circle ci.

When local is activated all folders in ./datasets are tested.

Important When adding a dataset, we should no longer upload it to AWS. The steps are:

1. Open a PR
2. Add a dataset as described in datasets/README.md
3. If all tests pass, push to master

Currently we have 49 functional datasets in our code base.

We have 6 datasets "under-construction" that don't pass the tests - so I put them in a folder "datasets_under_construction" - it would be nice to open a PR to fix them and put them in the datasets folder.

Important when running tests locally, the datasets are cached so to rerun them delete your local cache via:
rm -r ~/.cache/huggingface/datasets/*

@thomwolf @mariamabarham @lhoestq

[thomwolf]

Super nice, good job @patrickvonplaten!