[Tests] Local => aws

## Change default Test from local => aws

As a default we set` aws=True`, `Local=False`, `slow=False`

### 1. RUN_AWS=1 (default)
This runs 4 tests per dataset script.

a) Does the dataset script have a valid etag / Can it be reached on AWS? 
b) Can we load its `builder_class`?
c) Can we load **all** dataset configs?
d) _Most importantly_: Can we load the dataset? 

Important - we currently only test the first config of each dataset to reduce test time. Total test time is around 1min20s.

### 2. RUN_LOCAL=1 RUN_AWS=0

***This should be done when debugging dataset scripts of the ./datasets folder***

This only runs 1 test per dataset test, which is equivalent to aws d) - Can we load the dataset from the local `datasets` directory?

### 3. RUN_SLOW=1

We should set up to run these tests maybe 1 time per week ? @thomwolf 

The `slow` tests include two more important tests. 

e) Can we load the dataset with all possible configs? This test will probably fail at the moment because a lot of dummy data is missing. We should add the dummy data step by step to be sure that all configs work.

f) Test that the actual dataset can be loaded. This will take quite some time to run, but is important to make sure that the "real" data can be loaded. It will also test whether the dataset script has the correct checksums file which is currently not tested with `aws=True`. @lhoestq - is there an easy way to check cheaply whether the `dataset_info.json` is correct for each dataset script? 

For each dataset, If there exist a `dataset_info.json`, then the command `nlp-cli test path/to/my/dataset --al_configs` is successful only if the `dataset_infos.json` is correct. The infos are correct if the size and checksums of the downloaded file are correct, and if the number of examples in each split are correct.

Note: the `test` command is supposed to test the script, that's why it runs the script even if the cached files already exist. Let me know if it's good to you.

> For each dataset, If there exist a `dataset_info.json`, then the command `nlp-cli test path/to/my/dataset --al_configs` is successful only if the `dataset_infos.json` is correct. The infos are correct if the size and checksums of the downloaded file are correct, and if the number of examples in each split are correct.
> 
> Note: the `test` command is supposed to test the script, that's why it runs the script even if the cached files already exist. Let me know if it's good to you.

Does it have to download the whole data to check if the checksums are correct? I guess so no? 

> > For each dataset, If there exist a `dataset_info.json`, then the command `nlp-cli test path/to/my/dataset --al_configs` is successful only if the `dataset_infos.json` is correct. The infos are correct if the size and checksums of the downloaded file are correct, and if the number of examples in each split are correct.
> > Note: the `test` command is supposed to test the script, that's why it runs the script even if the cached files already exist. Let me know if it's good to you.
> 
> Does it have to download the whole data to check if the checksums are correct? I guess so no?

Yes it has to download them all (unless they were already downloaded in which case it just uses the cached downloaded files).