test: add test

test/integration/eslint/config-core-web-vitals/.eslintrc

@@ -0,0 +1,4 @@
+{
+  "extends": "next/core-web-vitals",
+  "root": true
+}

test/integration/eslint/config-core-web-vitals/pages/index.js

@@ -0,0 +1,9 @@
+const Home = () => (
+  <div>
+    <p>Home</p>
+    <script src="https://example.com" />
+    <img src="https://example.com/image.png" />
+  </div>
+)
+
+export default Home

test/integration/eslint/test/index.test.js

@@ -8,6 +8,7 @@ jest.setTimeout(1000 * 60 * 2)
 
 const dirFirstTimeSetup = join(__dirname, '../first-time-setup')
 const dirCustomConfig = join(__dirname, '../custom-config')
+const dirWebVitalsConfig = join(__dirname, '../config-core-web-vitals')
 const dirPluginRecommendedConfig = join(
   __dirname,
   '../plugin-recommended-config'
@@ -167,6 +168,21 @@ describe('ESLint', () => {
       )
     })
 
+    test('shows warnings and errors with next/core-web-vitals config', async () => {
+      const { stdout, stderr } = await nextLint(dirWebVitalsConfig, [], {
+        stdout: true,
+        stderr: true,
+      })
+
+      const output = stdout + stderr
+      expect(output).toContain(
+        "Warning: Do not use <img>. Use Image from 'next/image' instead."
+      )
+      expect(output).toContain(
+        'Error: External synchronous scripts are forbidden'
+      )
+    })
+
     test('shows warnings and errors when extending plugin recommended config', async () => {
       const { stdout, stderr } = await nextLint(
         dirPluginRecommendedConfig,