Add tests for dataset cards
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 