[test] partial coverage for train_mbart_enro_cc25.sh
CI covers all the examples logic that doesn't run on CUDA.
Adds coverage for:
- user facing bash script train_mbart_cc25_enro.sh
- the idea that seq2seq/finetune.py should lead to models getting better/ val BLEU increasing.

This only takes 10s to run on brutasse, so I gave it a minute for github actions CI, the only automated tester that will run this.

Will add coverage for `--do_predict` once [this](https://github.com/PyTorchLightning/pytorch-lightning/issues/2673) is fixed.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=h1) Report
> Merging [#5976](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2c0da7803a75f0fc6e6d484e23ca283faa32d785&el=desc) will **decrease** coverage by `1.38%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5976/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5976      +/-   ##
==========================================
- Coverage   78.66%   77.28%   -1.39%     
==========================================
  Files         146      146              
  Lines       26227    26227              
==========================================
- Hits        20632    20270     -362     
- Misses       5595     5957     +362     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5976/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5976/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5976/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5976/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5976/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=footer). Last update [2c0da78...f83fdd3](https://codecov.io/gh/huggingface/transformers/pull/5976?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
