Added tests for next/router in app directory (#39867)

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Added tests to ensure that the `next/router` returns null for `useRouter` and `withRouter` when used with the app directory.

test/e2e/app-dir/app/app/old-router/IsNull.js

@@ -0,0 +1,7 @@
+export default function IsNull({ value }) {
+  if (value === null) {
+    return <div className="was-null">Value Was Null</div>
+  }
+
+  return <div className="was-not-null">Value Was Not Null</div>
+}

test/e2e/app-dir/app/app/old-router/Router.client.js

@@ -0,0 +1,15 @@
+import { useRouter, withRouter } from 'next/router'
+import IsNull from './IsNull'
+
+function ClientRouter({ router: withRouter }) {
+  const router = useRouter()
+
+  return (
+    <>
+      <IsNull value={withRouter} />
+      <IsNull value={router} />
+    </>
+  )
+}
+
+export default withRouter(ClientRouter)

test/e2e/app-dir/app/app/old-router/Router.js

@@ -0,0 +1,19 @@
+import { useRouter, withRouter } from 'next/router'
+import IsNull from './IsNull'
+import ServerRouter from './Router.server'
+import ClientRouter from './Router.client'
+
+function SharedRouter({ router: withRouter }) {
+  const router = useRouter()
+
+  return (
+    <>
+      <IsNull value={withRouter} />
+      <IsNull value={router} />
+      <ServerRouter />
+      <ClientRouter />
+    </>
+  )
+}
+
+export default withRouter(SharedRouter)

test/e2e/app-dir/app/app/old-router/Router.server.js

@@ -0,0 +1,15 @@
+import { useRouter, withRouter } from 'next/router'
+import IsNull from './IsNull'
+
+function ServerRouter({ router: withRouter }) {
+  const router = useRouter()
+
+  return (
+    <>
+      <IsNull value={withRouter} />
+      <IsNull value={router} />
+    </>
+  )
+}
+
+export default withRouter(ServerRouter)

test/e2e/app-dir/app/app/old-router/page.server.js

@@ -0,0 +1,9 @@
+import Router from './Router'
+
+export default function Page() {
+  return (
+    <div id="old-router">
+      <Router />
+    </div>
+  )
+}

test/e2e/app-dir/index.test.ts

@@ -720,6 +720,24 @@ describe('app dir', () => {
         })
       })
 
+      describe('next/router', () => {
+        it('should always return null when accessed from /app', async () => {
+          const browser = await webdriver(next.url, '/old-router')
+
+          try {
+            await browser.waitForElementByCss('#old-router')
+
+            const notNull = await browser.elementsByCss('.was-not-null')
+            expect(notNull.length).toBe(0)
+
+            const wasNull = await browser.elementsByCss('.was-null')
+            expect(wasNull.length).toBe(6)
+          } finally {
+            await browser.close()
+          }
+        })
+      })
+
       describe('hooks', () => {
         describe('useCookies', () => {
           it('should retrive cookies in a server component', async () => {