MobileBERT tokenizer tests
# What does this PR do?

This PR implements tests for MobileBERT. As MobileBERT uses a copy of the BERT Tokenizer, the test inherits from BertTokenizationTest, and also checks the merge & vocab files for these two models are identical.

Contributes fixes to issue #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?

cc. @LysandreJik @SaulLu 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Obviously - thanks!<|||||>Hi, @leondz 

The main branch has recently merged a PR that changes test folders, like 
```
tests/mobilebert -> tests/models/mobilebert
```
Could you follow the ideas shown in the instructions in [this](https://github.com/huggingface/transformers/pull/17008#issuecomment-1116059265) to incorporate the changes into your working branch. Thank you. (You might need to fix a few import places)
<|||||>> Hi, @leondz
> 
> The main branch has recently merged a PR that changes test folders, like
> 
> ```
> tests/mobilebert -> tests/models/mobilebert
> ```
> 
> Could you follow the ideas shown in the instructions in [this](https://github.com/huggingface/transformers/pull/17008#issuecomment-1116059265) to incorporate the changes into your working branch. Thank you. (You might need to fix a few import places)

Thanks for this, it makes sense. By the way, `make fixup` seems to adjust content in /examples and /docs in a way that looks mistaken - out of scope for this PR but is that something to be looked at?

e.g.
```

--- a/docs/source/en/model_doc/bert-generation.mdx
+++ b/docs/source/en/model_doc/bert-generation.mdx
@@ -49,7 +49,7 @@ Usage:
 
 >>> input_ids = tokenizer(
 ...     "This is a long article to summarize", add_special_tokens=False, return_tensors="pt"
-... ).input_ids
+>>> ).input_ids
 >>> labels = tokenizer("This is a short summary", return_tensors="pt").input_ids
 

```<|||||>Could you check this comment

https://github.com/huggingface/transformers/pull/17008#issuecomment-1115007653

and see if it works well? That's my first thought :-)<|||||>> By the way, `make fixup` seems to adjust content in /examples and /docs in a way that looks mistaken - out of scope for this PR but is that something to be looked at?
> 
> e.g.
> 
> ```
> 
> --- a/docs/source/en/model_doc/bert-generation.mdx
> +++ b/docs/source/en/model_doc/bert-generation.mdx
> @@ -49,7 +49,7 @@ Usage:
>  
>  >>> input_ids = tokenizer(
>  ...     "This is a long article to summarize", add_special_tokens=False, return_tensors="pt"
> -... ).input_ids
> +>>> ).input_ids
>  >>> labels = tokenizer("This is a short summary", return_tensors="pt").input_ids
>  
> ```

OK this was fixed in https://github.com/huggingface/doc-builder/pull/207 :)




> Could you check this comment
> 
> [#17008 (comment)](https://github.com/huggingface/transformers/pull/17008#issuecomment-1115007653)
> 
> and see if it works well? That's my first thought :-)

Yes! All done :)