refactor deepfloyd if img2img superresolution pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the DeepFloyd IF img2img super-resolution pipeline tests to the new mixin structure from #14113, same shape as the previous IF ports: an IFImg2ImgSuperResolutionPipelineTesterConfig plus TestIFImg2ImgSuperResolutionPipeline(PipelineTesterMixin) and TestIFImg2ImgSuperResolutionPipelineMemory(PipelineOffloadTesterMixin), slow class ported to pytest, the stage-II component set inlined, and a fresh CPU slice in test_inference. original_image joins batch_input_params, as in the old file.

The same verified choices as #14580 apply: @skip_mps kept (all-NaN output on mps), latents dropped from optional_input_params, the custom test_save_load_optional_components ported (prompt pre-encoded), and no float16 override per the finding on #14596. Follows #14603.

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?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Discussed on Slack with @sayakpaul
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@sayakpaul