Rename test_feature_extraction files
# What does this PR do?

Renames the tests for image processors: `test_feature_extraction_xxx.py` -> `test_image_processing_xxx.py`

A follow up PR will change the feature extractor references to equivalent image processor ones in the files. 


## Before submitting
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- [ ] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you @amyeroberts !

I think there are a few lines in `tests/utils/test_add_new_model_like.py` to be changed, like

https://github.com/amyeroberts/transformers/blob/6b10c045e259a47f3786ceb11089fe418828346e/tests/utils/test_add_new_model_like.py#L62
<|||||>@ydshieh Done! 