Update xformers SD3 test

[DN6]

What does this PR do?

SD3 only supports using the JointAttnProcessor. xformers tests are failing because we're attempting to set a default attn processor (which the SD3 transformer does not support yet) and use the xFormersAttnProcessor with joint attention (also not supported by the SD3 transformer)
https://github.com/huggingface/diffusers/actions/runs/9667879876/job/26670701141#step:9:1

Skip these tests for SD3.

Fixes # (issue)

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[yiyixuxu]

thanks!