[AWS tests] AWS test should not run for canonical datasets
AWS tests should in general not run for canonical datasets. Only local tests will run in this case. This way a PR is able to pass when adding a new dataset.

This PR changes to logic to the following: 

1) All datasets that are present in `nlp/datasets` are tested only locally. This way when one adds a canonical dataset, the PR includes his dataset in the tests.

2) All datasets that are only present on AWS, such as `webis/tl_dr` atm are tested only on AWS. 

I think the testing structure might need a bigger refactoring and better documentation very soon. 

Merging for now to unblock new PRs @thomwolf @mariamabarham .