rename all test_processing_*.py to test_processor_*.py
Author: yonigozlan
Pull request: yes

# What does this PR do?
I noticed that we have `test_processor_*.py` for some models and `test_processing_*.py` for others, for no apparent reason (unless I'm mistaken). I converted all the `test_processing_*.py` to `test_processor_*.py` as `test_processor_*.py` are currently more prevalent. However it could make more sense to adopt `test_processing_*.py`, to be coherent with `test_image_processing_*.py,` so happy to change that if we think it's better.

Also this caused a bit of a duplication issue for paligemma processor tests, as it seems that @zucchini-nlp and I were working on processor tests at the same time, and both got merged, one as  `test_processor_paligemma.py` and the other as `test_processing_paligemma.py` 😅. I merged them both into `test_processor_paligemma.py`.

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