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

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

tests/dfx/perf/scripts/run_benchmark.py

@@ -8,7 +8,6 @@ from typing import Any
 
 import pytest
 
-from tests.conftest import OmniServer
 from tests.dfx.conftest import (
     create_benchmark_indices,
     create_test_parameter_mapping,
@@ -16,6 +15,7 @@ from tests.dfx.conftest import (
     get_benchmark_params_for_server,
     load_configs,
 )
+from tests.helpers.runtime import OmniServer
 
 os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
 os.environ["VLLM_TEST_CLEAN_GPU_MEMORY"] = "0"