[MarianTokenizer] implement save_vocabulary and other common methods
- adds `test_tokenization_marian.py` which runs the common tokenizer tests.
- adds `save_vocabulary` and other methods to `MarianTokenizer` to make the common tests pass.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=h1) Report
> Merging [#4389](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/48c3a70b4eaedab1dd9ad49990cfaa4d6cb8f6a0&el=desc) will **decrease** coverage by `0.37%`.
> The diff coverage is `96.22%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4389/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4389      +/-   ##
==========================================
- Coverage   78.41%   78.03%   -0.38%     
==========================================
  Files         123      123              
  Lines       20432    20477      +45     
==========================================
- Hits        16021    15980      -41     
- Misses       4411     4497      +86     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/4389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.10% <96.22%> (+5.77%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-28.03%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=footer). Last update [48c3a70...1bd2df5](https://codecov.io/gh/huggingface/transformers/pull/4389?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Can anyone help with this issue: #5040 ?