Fix get_dummy_inputs for Stable Diffusion Inpaint Tests (#4845)
* Change StableDiffusionInpaintPipelineFastTests.get_dummy_inputs to produce a random image and a white mask_image.
* Add dummy expected slices for the test_stable_diffusion_inpaint tests.
* Remove print statement
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Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>