Made MaskFormerModelTest faster
# What does this PR do?
This PR changes the configuration inside `tests/maskformer/test_modeling_maskformer.py::MaskFormerModelTest` creating a smaller model to make the test run faster
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15942). All of your documentation changes will be reflected on that endpoint.