add test checking the offsets for an input splitted into words for different `add_prefix_space` and `trim_offsets` args
Author: SaulLu
Pull request: yes

# What does this PR do?

Added a new test that tests the offsets returned by the fast tokenizers when the input is already pre-tokenized for the roberta tokenizer. To pass this test use the new version of the library tokenizers v0.11.0 (thanks to [this PR](https://github.com/huggingface/tokenizers/pull/844)).
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This addition was discussed [here](https://github.com/huggingface/transformers/pull/14752#issuecomment-998144055)


## 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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Comments:
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