Add missing tokenizer tests - Longformer
# What does this PR do?

This PR add tests for Longformer tokenizer copying tests from Roberta tokenizer's test suite, because those tokenizers are absolutely identical. 

Fixes #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.
- [ ] 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?
@SaulLu @LysandreJik 


_The documentation is not available anymore as the PR was closed or merged._<|||||>I read discussion in merged tokenizers' tests PRs and post [~~Don't~~ Repeat Yourself*](https://huggingface.co/blog/transformers-design-philosophy) on HF blog and I manually add "the copying mechanism". But I don't understand how it is work, so I tried not to change copied test code from Roberta tokenizer tests. If code modification is not a problem, I would like to add some minor changes, e.g. delete commented code and split big test into smaller one.
Could describe "copying mechanism" works in more details?<|||||>Thanks a lot for working on this @tgadeliya!! 

As far as I know, there are no identified "practices" for this case (cc @LysandreJik in case you have another opinion). Nevertheless, if changes are relevant, they are obviously welcome. For example, it is possible to indicate the changes made as here: 
https://github.com/huggingface/transformers/blob/d95a32cc60e5d92b4bf08cd805c6b0db7b4100cc/src/transformers/models/deberta/modeling_deberta.py#L308-L309
If the differences are too long to list perhaps the message can just explain why it diverged from the originally copied and pasted code.

Does this help you?<|||||>@SaulLu, Sorry for the late reply. Summer is ending :) 

Thanks for your comment. Now it is clear for me. Actually, I came to the conclusion, that code cleaning not so necessary considering all pros and cons. So this PR can be reviewed and merged <|||||>@SaulLu  I refreshed this PR, so now it is ready to merge<|||||>Thanks @tgadeliya :hugs: 