[Tests] Add tiny model test for StableDiffusionXLPipeline

Covers base text-to-image plus CPU offload; UNet stays full-size
since its architecture is hardcoded rather than config-driven.

Co-Authored-By: Claude <noreply@anthropic.com>

Signed-off-by: Nick Cao <ncao@redhat.com>

tests/model_tests/diffusion/test_alignment.py

@@ -84,7 +84,6 @@ EXCLUDED_MODELS = [
     "HiDreamImagePipeline",
     "HiDreamO1ImagePipeline",
     "DreamZeroPipeline",
-    "StableDiffusionXLPipeline",
     "Gr00tN1d7Pipeline",
     "Pi0Pipeline",
     "Pi05Pipeline",