Add failing spec for app.exit with >2 windows

spec/fixtures/api/exit-closes-all-windows-app/main.js

@@ -0,0 +1,19 @@
+const {app, BrowserWindow} = require('electron')
+
+const windows = []
+
+function createWindow (id) {
+  const window = new BrowserWindow({show: false})
+  window.loadURL(`data:,window${id}`)
+  windows.push(window)
+}
+
+app.once('ready', () => {
+  for (let i = 1; i <= 5; i++) {
+    createWindow(i)
+  }
+
+  setImmediate(function () {
+    app.exit(123)
+  })
+})