[ci] add vllm_test_utils (#10659)

Signed-off-by: youkaichao <youkaichao@gmail.com>

Dockerfile.cpu

@@ -62,4 +62,8 @@ WORKDIR /workspace/
 
 RUN ln -s /workspace/vllm/tests && ln -s /workspace/vllm/examples && ln -s /workspace/vllm/benchmarks
 
+# install development dependencies (for testing)
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install -e tests/vllm_test_utils
+
 ENTRYPOINT ["python3", "-m", "vllm.entrypoints.openai.api_server"]