Move tests/utils.py -> transformers/testing_utils.py
Fixes #5350 
The motivation is to allow examples/ tests to use these utilities.

for both groups of tests, the import is
```python
from transformers.testing_utils import slow
```

Motivation: I was about to rewrite the @slow decorator today and felt that this was cleaner.
CI failure is spurious<|||||>no objection<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5350?src=pr&el=h1) Report
> Merging [#5350](https://codecov.io/gh/huggingface/transformers/pull/5350?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/28a690a80e6c8dbcb50b5628ef853146e1940125&el=desc) will **increase** coverage by `1.86%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5350/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5350?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5350      +/-   ##
==========================================
+ Coverage   76.18%   78.04%   +1.86%     
==========================================
  Files         138      139       +1     
  Lines       24292    24339      +47     
==========================================
+ Hits        18506    18996     +490     
+ Misses       5786     5343     -443     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5350?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `80.85% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.92% <0.00%> (-68.47%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.47% <0.00%> (-49.57%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.95% <0.00%> (-0.59%)` | :arrow_down: |
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `57.27% <0.00%> (+24.54%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.75% <0.00%> (+25.89%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `75.98% <0.00%> (+55.48%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5350/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.72% <0.00%> (+73.10%)` | :arrow_up: |

------

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