add a template to add missing tokenization test
# What does this PR do?

In this PR I propose to add a cookie cutter template for tokenization tests. 

It is a first version and could be useful to propose in a good first issue to users to add the missing tests to the tokenizers of the following models:
- Flaubert #15137
- LED
- RemBert
- Splinter

 and eventually for these tokenizers too (currently they just inherit from `BertTokenizer` and just re-define the attributes):
- MobileBert
- ConvBert
- Electra
- Longformer
- RetriBert

I plan to give a little more information on how to add the test to the good first issue ticket than is currently shown in the readme. But don't hesitate if you think it is better to say as much as possible in the current readme

## 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?
- [ ] 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.
- [ ] 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).
- [ ] 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. I would love to have your input on this @LysandreJik , @sgugger , @patrickvonplaten or @patil-suraj  :hugs: 


_The documentation is not available anymore as the PR was closed or merged._