refactor wan vace pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the wan VACE pipeline tests to the new mixin structure from #14113: a WanVACEPipelineTesterConfig plus TestWanVACEPipeline(PipelineTesterMixin) and TestWanVACEPipelineMemory(MemoryTesterMixin).

Existing coverage is kept: test_inference, the single and multiple reference-image variants, the two-transformer variants, and test_save_load_optional_components. The two batch tests stay skipped because WanVACEPipeline does not support batched prompts (it raises on a list of prompts). 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