Fix StableDiffusionInstructPix2PixPipelineSingleFileSlowTests

[hlky]

What does this PR do?

https://github.com/huggingface/diffusers/actions/runs/12738450237/job/35500790623

We add instruct-pix2pix to DIFFUSERS_TO_LDM_DEFAULT_IMAGE_SIZE_MAP and introduce single_file_kwargs to SDSingleFileTesterMixin, this is to pass extract_ema=True to convert_ldm_unet_checkpoint and match inference results with the checkpoint at timbrooks/instruct-pix2pix which is created from extract_ema=True.

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