[Benchmark] Correctly kill vllm process in performance benchamrk (#2782)

### What this PR does / why we need it?
vLLM now names the process with VLLM prefix after
https://github.com/vllm-project/vllm/pull/21445, we should kill the
correct process name after one iteration benchmark to avoid OOM issue
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.10.1.1
- vLLM main:
e599e2c65e

---------

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

benchmarks/tests/serving-tests.json

@@ -23,7 +23,8 @@
       "hf_split": "train",
       "endpoint": "/v1/chat/completions",
       "dataset_path": "lmarena-ai/vision-arena-bench-v0.1",
-      "num_prompts": 200
+      "num_prompts": 200,
+      "no_stream": ""
     }
   },
   {