Add failing spec for fetch from isolated world

spec/fixtures/api/isolated-preload.js

@@ -1,9 +1,15 @@
+// Ensure fetch works from isolated world origin
+fetch('http://localhost:1234')
+fetch('https://localhost:1234')
+fetch(`file://${__filename}`)
+
 const {ipcRenderer, webFrame} = require('electron')
 
 window.foo = 3
 
 webFrame.executeJavaScript('window.preloadExecuteJavaScriptProperty = 1234;')
 
+
 window.addEventListener('message', (event) => {
   ipcRenderer.send('isolated-world', {
     preloadContext: {