refactor wan image-to-video pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the wan image-to-video pipeline tests to the new mixin structure from #14113. The file covers two pipelines, so it keeps two config/test/memory triads: WanImageToVideoPipeline and the first-last-frame WanFLFToVideoPipeline.

Existing coverage is kept for both: test_inference and test_save_load_optional_components (overridden to null only transformer_2, since _optional_components also lists the required transformer). No caching tests are added since the old suite had none, and the attention-slicing test is dropped as the new framework has no equivalent.

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

[sayakpaul]

Suggested change

	from ...testing_utils import (
	assert_tensors_close,
	torch_device,
	)
	from ...testing_utils import assert_tensors_close torch_device

[sayakpaul]

Suggested change

	from ..testing_utils import (
	BasePipelineTesterConfig,
	MemoryTesterMixin,
	PipelineTesterMixin,
	)
	from ..testing_utils import (
	BasePipelineTesterConfig, MemoryTesterMixin, PipelineTesterMixin,
	)

[akshan-main]

done

[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.