Moving `translation` pipeline to new testing scheme.
Author: Narsil
Pull request: yes

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Fixes # (issue)


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Models:

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Comments:
It seems one of the tests will need to be adapted: 
```
=================================== FAILURES ===================================
_____________ MBartEnroIntegrationTest.test_tokenizer_translation ______________
[gw1] linux -- Python 3.7.11 /usr/local/bin/python

self = <tests.test_tokenization_mbart.MBartEnroIntegrationTest testMethod=test_tokenizer_translation>

    @require_torch
    def test_tokenizer_translation(self):
>       inputs = self.tokenizer._build_translation_inputs("A test", src_lang="en_XX", tgt_lang="ar_AR")
E       TypeError: _build_translation_inputs() missing 1 required positional argument: 'return_tensors'
```

Updated the test !