chore: refactor context isolation spec (#14394)

* spec: refactor BrowserWindow module contextIsolation option

* spec: check for serialzation in isolated renderers separately

spec/fixtures/api/isolated-fetch-preload.js

@@ -0,0 +1,6 @@
+const {ipcRenderer} = require('electron')
+
+// Ensure fetch works from isolated world origin
+fetch('https://localhost:1234').catch(err => {
+  ipcRenderer.send('isolated-fetch-error', err.message)
+})