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

[akshan-main]

What does this PR do?

Refactors the DeepFloyd IF inpainting super-resolution pipeline tests to the new mixin structure from #14113, same shape as the previous IF ports: an IFInpaintingSuperResolutionPipelineTesterConfig plus TestIFInpaintingSuperResolutionPipeline(PipelineTesterMixin) and TestIFInpaintingSuperResolutionPipelineMemory(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. This is the last IF file; the now-unused shared IFPipelineTesterMixin in the package __init__.py gets removed once #14604 lands, since the old img2img superresolution file still imports it until then. Follows #14604.

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

[github-actions]

Hi @akshan-main, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

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