[run_seq2seq.py] restore functionality: saving to test_generations.txt
This PR restores the original functionality that for some reason was modified.

Fixes: https://github.com/huggingface/transformers/issues/10381

@sgugger

Ah, I re-read it closer, you're correct. I just remembered the part about the `test_generations.txt` but didn't bother to check out the full story. My bad. I will study it and follow up once I understand it better. <|||||>I went back to `finetune_trainer.py` from December and checked that it was just saving `test_generations.txt` at the very end once. I can't find any code where it did generate this at every checkpoint. @kingpalethe, please correct me if I'm wrong.

If there were to be a `save_checkpoint` callback then it could generate one for each saved check point. So it probably needs to be requested via a feature request Issue.

So the current PR is still is a good idea to support those who relied on this particular filename. But I'm not attached to it.<|||||>I don't think it matters much, we just have to be quick if people using the new script start to rely on the new name.<|||||>OK, let's restore the original name. 