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

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

tests/diffusion/test_diffusion_model_runner.py

@@ -8,7 +8,7 @@ import pytest
 import torch
 
 import vllm_omni.diffusion.worker.diffusion_model_runner as model_runner_module
-from tests.utils import hardware_test
+from tests.helpers.mark import hardware_test
 from vllm_omni.diffusion.worker.diffusion_model_runner import DiffusionModelRunner
 
 pytestmark = [pytest.mark.diffusion]