migrate cogvideox lora tests to the pipeline-level mixins

[akshan-main]

What does this PR do?

Migrates the CogVideoX pipeline LoRA tests from tests/lora/test_lora_layers_cogvideox.py to the pipeline-level mixins from #14268, same shape as #14613: TestCogVideoXPipelineLoRA(CogVideoXPipelineTesterConfig, LoraTesterMixin) and TestCogVideoXPipelineLoRAMemory(CogVideoXPipelineTesterConfig, LoraMemoryTesterMixin) in tests/pipelines/cogvideo/test_cogvideox.py, and the old file removed.

The old file's specifics dissolve into the new structure the same way: text-encoder tests skip through _lora_loadable_modules, the three "Not supported in CogVideoX" skips disappear since those tests live in UNetLoraTesterMixin, the group-offloading gating is handled by the memory mixin, and the 9e-3 tolerance overrides are not needed at the mixin defaults (verified by running the classes). Follows #14614.

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]

/diffusers-bot pytest tests/pipelines/cogvideo/test_cogvideox.py -k "lora"

[github-actions]

✅ pytest tests/pipelines/cogvideo/test_cogvideox.py -k "lora" passed on GPU — view logs.