[Tests] add slow tests
This PR adds a slow test that downloads the "real" dataset. The test is decorated as "slow" so that it will not automatically run on circle ci.

Before uploading a dataset, one should test that this test passes, manually by running 

```
RUN_SLOW=1 pytest tests/test_dataset_common.py::DatasetTest::test_load_real_dataset_<your-dataset-script-name>
```

This PR should be merged after PR: #33 