[Tests] refactor tests
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Nice !
This PR separates AWS and Local tests to remove these ugly statements in the script:
```python
        if "/" not in dataset_name:
            logging.info("Skip {} because it is a canonical dataset")
            return
```

To run a `aws` test, one should now run the following command: 

```python 
pytest -s tests/test_dataset_common.py::AWSDatasetTest::test_builder_class_wmt14
```

The same `local` test, can be run with:
```python 
pytest -s tests/test_dataset_common.py::LocalDatasetTest::test_builder_class_wmt14
```
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