refactor cogvideox image to video pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the CogVideoX image-to-video pipeline tests to the new mixin structure from #14113: a CogVideoXImageToVideoPipelineTesterConfig plus TestCogVideoXImageToVideoPipeline(PipelineTesterMixin) and TestCogVideoXImageToVideoPipelineMemory(MemoryTesterMixin), with the integration test moved to a pytest slow class. Follows #14276.

Existing coverage is kept: test_inference (now asserting a real value slice), test_vae_tiling (with its transformer re-init for the learned positional embeddings), test_fused_qkv_projections, and the batch-single tolerance override. The attention-slicing and test_callback_inputs overrides are dropped since the base mixin covers them. No caching tests are added since the old suite had none.

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.