Ensure testing MobileNetV2 image processor for both slow and fast

[namgyu-youn]

What does this PR do?

• Remove test skipping for importing is_torch_available & is_vision_available, because test result without it doesn't make sense.
• Incline (run directly) unittest for MobileNetV2ImageProcessor and MobileNetV2ImageProcessorFast
• Related Issue/PR: add general hub test for Fast Image Processors in test_image_processing_utils #40086

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
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• Was this discussed/approved via a Github issue or the forum? Please add a link
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

cc @yonigozlan

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: mobilenet_v2