[ci] add vllm_test_utils (#10659)

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

Dockerfile.ppc64le

@@ -29,6 +29,9 @@ RUN --mount=type=cache,target=/root/.cache/pip  \
 RUN --mount=type=bind,source=.git,target=.git \
     VLLM_TARGET_DEVICE=cpu python3 setup.py install
 
+# install development dependencies (for testing)
+RUN python3 -m pip install -e tests/vllm_test_utils
+
 WORKDIR /workspace/
 
 RUN ln -s /workspace/vllm/tests && ln -s /workspace/vllm/examples && ln -s /workspace/vllm/benchmarks