[Doctest] add installation doctest (#1179)

### What this PR does / why we need it?
Install doctest

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
CI passed

Related: https://github.com/vllm-project/vllm-ascend/pull/983

Co-authored-by: wangli <wangli858794774@gmail.com>

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>
Co-authored-by: wangli <wangli858794774@gmail.com>

tests/e2e/doctests/001-quickstart-test.sh

@@ -23,13 +23,11 @@ function simple_test() {
 }
 
 function quickstart_offline_test() {
-  export VLLM_USE_MODELSCOPE=true
   # Do real script test
   python3 "${SCRIPT_DIR}/../../examples/offline_inference_npu.py"
 }
 
 function quickstart_online_test() {
-  export VLLM_USE_MODELSCOPE=true
   vllm serve Qwen/Qwen2.5-0.5B-Instruct &
   wait_url_ready "vllm serve" "localhost:8000/v1/models"
   # Do real curl test