[s2s] test_bash_script.py - actually learn something
As discussed in https://github.com/huggingface/transformers/issues/6049 this PR replaces the original `test_train_mbart_cc25_enro_script` test with mostly new guts doing the following:

* [x] switches to downloading and caching a bigger custom dataset, which is a subset of the full wmt_en_ro.  I created https://cdn-datasets.huggingface.co/translation/wmt_en_ro-tr40k-va0.5k-te0.5k.tar.gz - hope the name is intuitive - self-documenting. It's just 3.6M (vs 56M original). I made it using this script: https://github.com/stas00/porting/blob/master/transformers/translation/make-wmt_en_ro-subset.md

* [x] performs qualitative checks on the eval/test results:
   - Minimum learning requirement 1: BLEU improves over the course of training by more than 2 pts
   - Minimum learning requirement 2: BLEU finishes above 17
   - Minimum learning requirement 3: test bleu and val bleu within ~1 pt. 

also makes some extra improvements:
* [x] uses `require_torch_gpu` decorator
* [x] removes hardcoded paths

Runtime is about ~2min on unoptimized rtx-3090.

Fixes: #6049

@sshleifer 
It's ready to merge