Add tests for downloading private HuggingFace models

[sjrl]

Is your feature request related to a problem? Please describe. We would like to add multiple test cases to make sure a problem with Hugging Face authorization tokens is recognized faster in the future. The deepset auth token could be stored as a GitHub secret to allow us to test the download of private HF models.

Describe the solution you'd like Test the downloading of private Hugging Face models for our various Node types throughout Haystack. For example, we could test the reader node with something like

from haystack.nodes import FARMReader
reader = FARMReader(model_name_or_path="private/model", use_gpu=True, use_auth_token=True)

This would require us to add the deepset Hugging Face API token to GitHub secrets and for us to add multiple private models on Hugging Face.