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

[akshan-main]

What does this PR do?

Refactors the wan video-to-video pipeline tests to the new mixin structure from #14113: a WanVideoToVideoPipelineTesterConfig plus TestWanVideoToVideoPipeline(PipelineTesterMixin) and TestWanVideoToVideoPipelineMemory(MemoryTesterMixin).

Existing coverage is kept: test_inference, with the video input batched in the batch tests. The two fp16 tests stay skipped because the pipeline has to run in mixed precision (casting the whole pipeline to fp16 errors). 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

[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.