add preload.js script to electron main

tests/unit/electron.spec.js

@@ -0,0 +1,32 @@
+/**
+ * @jest-environment node
+ */
+import spectron from 'spectron'
+import { testWithSpectron } from 'vue-cli-plugin-electron-builder'
+jest.setTimeout(50000)
+
+test('Window Loads Properly', async () => {
+  // Wait for dev server to start
+  const { app, stopServe } = await testWithSpectron(spectron)
+  const win = app.browserWindow
+  const client = app.client
+
+  // Window was created
+  expect(await client.getWindowCount()).toBe(1)
+  // It is not minimized
+  expect(await win.isMinimized()).toBe(false)
+  // Window is visible
+  expect(await win.isVisible()).toBe(true)
+  // Size is correct
+  const { width, height } = await win.getBounds()
+  expect(width).toBeGreaterThan(0)
+  expect(height).toBeGreaterThan(0)
+  // App is loaded properly
+  expect(
+    /Welcome to Your Vue\.js (\+ TypeScript )?App/.test(
+      await client.getHTML('#app')
+    )
+  ).toBe(true)
+
+  await stopServe()
+})