Add some tests for check_dummies
# What does this PR do?

This PR adds a new subfolder in the `tests` folder for the tests of the quality scripts used in the CI (so not inside the Transformers lib). As seen recently with the `check_dummies` script, when people apply too many modifications to our repo and update those scripts, there might be some breaking changes that slip through the cracks. Hopefully such new tests will help catch the failures early.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19146). All of your documentation changes will be reflected on that endpoint.