Add hint to install pytest-xdist
Just a small hint that pytest-xdist should be installed before running the make test step
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2315?src=pr&el=h1) Report
> Merging [#2315](https://codecov.io/gh/huggingface/transformers/pull/2315?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/aeef4823ab6099249679756182700e6800024c36?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2315/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2315?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2315   +/-   ##
=======================================
  Coverage   73.49%   73.49%           
=======================================
  Files          87       87           
  Lines       14793    14793           
=======================================
  Hits        10872    10872           
  Misses       3921     3921
```



------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2315?src=pr&el=footer). Last update [aeef482...4c48701](https://codecov.io/gh/huggingface/transformers/pull/2315?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>They're installed by `pip install -e .[dev]`. You don't have them because you modified that step. Let's discuss on #2316.