Add some tests for "sandbox" option

spec/fixtures/module/preload-sandbox.js

@@ -0,0 +1,13 @@
+(function () {
+  const {ipcRenderer} = require('electron')
+  window.ipcRenderer = ipcRenderer
+  if (location.protocol === 'file:') {
+    window.test = 'preload'
+    window.require = require
+    window.process = process
+  } else if (location.href !== 'about:blank') {
+    addEventListener('DOMContentLoaded', () => {
+      ipcRenderer.send('child-loaded', window.opener == null, document.body.innerHTML)
+    }, false)
+  }
+})()