Add camembert integration tests
Add integration tests for camembert comparing results to original fairseq code. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3375?src=pr&el=h1) Report
> Merging [#3375](https://codecov.io/gh/huggingface/transformers/pull/3375?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8becb732931bbab5dd75cca5f5e7c75b2516d10b&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3375/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3375?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3375      +/-   ##
==========================================
+ Coverage   77.64%   77.71%   +0.06%     
==========================================
  Files         100      100              
  Lines       16979    16979              
==========================================
+ Hits        13184    13195      +11     
+ Misses       3795     3784      -11     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3375?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3375/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.50% <0.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3375/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.52% <0.00%> (+1.73%)` | :arrow_up: |

------

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