[Tests] Test files locally
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Super nice, good job @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 
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