refactor deepfloyd if inpainting pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the DeepFloyd IF inpainting pipeline tests to the new mixin structure from #14113, same shape as the previous IF ports: an IFInpaintingPipelineTesterConfig plus TestIFInpaintingPipeline(PipelineTesterMixin) and TestIFInpaintingPipelineMemory(PipelineOffloadTesterMixin), slow class ported to pytest, dummy components inlined from the shared package mixin, and a fresh CPU slice in test_inference.

The same verified choices as #14580 apply: @skip_mps kept (all-NaN output on mps), latents dropped from optional_input_params, and the custom test_save_load_optional_components ported (prompt pre-encoded). The float16 override is not carried over, per the finding on #14596 that the eval-mode load removes the old non-determinism. Follows #14596.

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