Add missing Flaubert tokenizer tests

[bastrob]

What does this PR do?

This PR implements tests for Flaubert Tokenization part. It contributes to the issue referred in #16627

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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@ArthurZucker