[1/N][CI] Refactor accuracy test (#5400)

### What this PR does / why we need it?
1. Accuracy testing no longer compares eager and graph modes; instead,
it directly extracts the golden result under the graph mode
configuration (the implicit purpose of this case is to verify whether
modifications affect existing results)
2. Next step: finer-grained supervision of logits/sampler results
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: release/v0.13.0
- vLLM main:
254f6b9867

Signed-off-by: wangli <wangli858794774@gmail.com>

tests/e2e/multicard/2-cards/test_shared_expert_dp.py

@@ -9,7 +9,6 @@ from tests.e2e.model_utils import check_outputs_equal
 MODELS = [
     "deepseek-ai/DeepSeek-V2-Lite",
 ]
-os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
 
 
 @pytest.mark.parametrize("model", MODELS)