Add missing RetriBERT tokenizer tests
# What does this PR do?

Addresses issue [#16627](https://github.com/huggingface/transformers/issues/16627).


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] 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
members/contributors who may be interested in your PR.

@LysandreJik
@SaulLu 

## Notes

1. There was no folder `tests/retribert/` yet, so I created one and put an `__init__.py` in it. Is there anything else I have to do for these tests to get picked up by CI?

2. `RetriBertTokenizer` is identical to `BertTokenizer`, so I mostly just duplicated `BertTokenizationTest`. Is that fine or should I rather a) write new tests from scratch or b) figure out a way to reuse the code in `BertTokenizationTest`?
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @SaulLu, can you give me some guidance on how to proceed?

The CI is throwing the following error:
```Make sure the names of these test files match the name of the module or utils they are testing, or adapt the constant `SPECIAL_MODULE_TO_TEST_MAP` in `utils/tests_fetcher.py` to add them. If your test file is triggered separately and is not supposed to be run by the regular CI, add it to the `EXPECTED_TEST_FILES_NEVER_TOUCHED` constant instead.```

However, I think the naming is correct (`test_tokenization_retribert.py` tests `tokenization_retribert.py`) and adding it to neither constants makes sense to me. I'm also unable to reproduce this locally with `pytest`. Do you have any idea what triggered this?<|||||>The CI error seems to me to come from the fact that 3 days ago there was a re-organisation of the test folder (https://github.com/huggingface/transformers/pull/17034). 

To solves this, I suggest to1) merge the latest changes to main in your branch and 2) move the tests you added to conform to the new organisation (`tests/retribert` -> `tests/models/retribert`).

Keep me updated! :smile: 

<|||||>Hi @SaulLu, thank you very much for the reply. I think I've got it now! Can you take another look?<|||||>My pleasure! I would like to keep contributing and I was wondering if you could help me with a question related to that @SaulLu. 

I noticed that the `RetriBERT` model itself is missing test files as well and I would like to write those. How do I make sure that no one else is writing them concurrently? Do I open an issue or perhaps a WIP pull request? I have already checked that there currently is no open issue or pull request related to this.