Support multiple choice in tf common model tests
This the same as #4886, but for tensorflow (first time ever of me coding in tf!)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=h1) Report
> Merging [#4920](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5d63ca6c38cc0f583cdec4c3efcfce13c0a41fdc&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4920/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4920      +/-   ##
==========================================
- Coverage   77.10%   77.05%   -0.05%     
==========================================
  Files         128      128              
  Lines       21617    21618       +1     
==========================================
- Hits        16667    16657      -10     
- Misses       4950     4961      +11     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4920/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `95.16% <100.00%> (+0.89%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/4920/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `76.31% <0.00%> (-2.31%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4920/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.49% <0.00%> (-0.12%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=footer). Last update [5d63ca6...05e5aa7](https://codecov.io/gh/huggingface/transformers/pull/4920?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Very clean. LGTM!