[tests] failing test only when run in a group
If someone wants to solve a puzzle, this test:

```
RUN_SLOW=1 pytest examples/seq2seq/test_finetune_trainer.py::TestFinetuneTrainer::test_finetune_trainer_slow
```
works on its own, but fails if it's run in the group with other tests:
```
RUN_SLOW=1 pytest examples/seq2seq/test_finetune_trainer.py
```
it doesn't learn anything - eval_blue remains 0.0

The only small issue is that the test is being renamed and moved to use `run_seq2seq.py`, so if you're reading this in a few days, most likely it will be the following case instead - which has the exact same problem:

```
RUN_SLOW=1 pytest examples/tests/trainer/test_trainer_ext.py::TestTrainerExt::test_run_seq2seq_slow
```
works on its own, but fails if it's run in the group with other tests:
```
RUN_SLOW=1 pytest examples/tests/trainer/test_trainer_ext.py
```
it doesn't learn anything - eval_blue remains 0.0

Thanks.
Hi @stas00,

I could not understand how to use `RUN_SLOW` in the windows command line, When I run it I was getting
```
'RUN_SLOW' is not recognized as an internal or external command,
operable program or batch file.
```

It was mentioned in the contribution guidelines but I don't know how to enable it<|||||>Sorry, I don't know much about windows. 

Aren't you using some unixy shell on windows to run this? In which case it should support it?

Otherwise look up how to setup env vars in your windows shell.

And of course the simplest hack is for the duration of your test to simply comment out the `@slow` decorator inside the test file.
<|||||>oh, but I actually just fixed it here https://github.com/huggingface/transformers/pull/10584 - thank you for unearthing this one. I can close it now.