[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/common.sh

@@ -45,7 +45,30 @@ function wait_for_exit() {
     _info "===> Wait for ${VLLM_PID} to exit."
     sleep 1
   done
-  _info "===> Wait for ${VLLM_PID} to exit."
+  _info "===> Process ${VLLM_PID} has exited."
+}
+
+VENV_PATH=/tmp/vllm_venv
+
+function clean_venv() {
+  if [[ -n "$VENV_PATH" && -d "$VENV_PATH" ]]; then
+    _info "Cleaning up default virtual env path: ${VENV_PATH}"
+    deactivate || true
+    rm -rf "$VENV_PATH"
+  fi
+}
+
+function create_vllm_venv() {
+  # make a clean env path
+  clean_venv
+  _info "Creating vllm virtual environment at ${VENV_PATH}"
+  python3 -m venv ${VENV_PATH}
+  source ${VENV_PATH}/bin/activate
+}
+
+function get_version() {
+  local VERSION_NAME="$1"
+  python3 "${SCRIPT_DIR}/../../docs/source/conf.py" | jq .${VERSION_NAME} | tr -d '"'
 }
 
 SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)