test: refactor test:demo script into demo/*/test.sh

demo/nodejs/package.json

@@ -4,6 +4,7 @@
   "private": true,
   "main": "index.js",
   "scripts": {
+    "test": "./test.sh",
     "start": "node index.js"
   },
   "dependencies": {

demo/nodejs/test.sh

@@ -0,0 +1,3 @@
+set -ex
+
+npm start | grep 'NodeJS Demo for LiquidJS'