Add tests for dataset cards
@lhoestq

Should I remove the scripts? or atleast remove running them from the CircleCI config?

Also, I hope it is okay that the combined method (metadata+content) is only a slow test, and for the Circle CI, I assume only non-slow tests are run? If yes, this would mean separate tests for content and metadata. Also feel free to remove the scripts from the CI and also remove the scripts files :)
Adding tests for dataset cards

This PR will potentially remove the scripts being used for dataset tags and readme validation.

Additionally, this will allow testing dataset readmes by providing the name as follows:

```bash
pytest tests/test_dataset_cards.py::test_dataset_tags[fashion_mnist]
```
and

```bash
pytest tests/test_dataset_cards.py::test_readme_content[fashion_mnist]
```
or a combined test as:

```bash
pytest tests/test_dataset_cards.py::test_dataset_card[fashion_mnist]
```
@lhoestq 