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

demo/webpack/package.json

@@ -5,6 +5,7 @@
   "main": "dist/index.js",
   "scripts": {
     "start": "node dist/index.js",
+    "test": "./test.sh",
     "build": "webpack"
   },
   "dependencies": {

demo/webpack/test.sh

@@ -0,0 +1,4 @@
+set -ex
+
+npm run build
+npm start | grep 'Webpack Demo for LiquidJS'