fix flax example tests
# What does this PR do?

The `conftest.py` was missing from the flax examples directory. so the flax example tests are failing on master.
(cf [here](https://app.circleci.com/pipelines/github/huggingface/transformers/30924/workflows/7c0b3b8b-d664-4dc2-9c7e-69520347f0d5/jobs/317301))

This PR adds the `conftest.py` file in the flax example directory.