[CI] Restructure vLLM-Omni Test Layout, Fixture Scope, and Support Modules (#2620)

Signed-off-by: wangyu <410167048@qq.com>

tests/diffusion/cache/test_teacache_extractors.py

@@ -21,7 +21,7 @@ from unittest.mock import MagicMock, Mock, patch
 import pytest
 import torch
 
-from tests.utils import hardware_test
+from tests.helpers.mark import hardware_test
 from vllm_omni.diffusion.cache.teacache.extractors import extract_flux2_context, extract_flux2_klein_context
 from vllm_omni.diffusion.models.flux2_klein.flux2_klein_transformer import (
     Flux2Transformer2DModel,