use assert_tensors_close in the migrated pipeline tests

[akshan-main]

What does this PR do?

Switches the value compares in the migrated wan, cogvideo and qwenimage pipeline tests from torch.allclose / np.allclose to assert_tensors_close, as discussed in #14344. Also flattens two wan 2.2 expected slices that were committed with extra nesting and only passed through torch.allclose broadcasting.

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.