Correct statusCode when visiting _error directly (vercel#26610)

This fixes non-stop reloading when visiting `_error` directly in development caused by the `statusCode` being 200 unexpectedly while HMR returns the page as `invalid` which triggers `on-demand-entries` to reload the page. 

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added

Fixes: vercel#8036
x-ref: vercel#8033

packages/next/next-server/server/next-server.ts

@@ -1444,6 +1444,7 @@ export default class Server {
   ): Promise<string | null> {
     const is404Page = pathname === '/404'
     const is500Page = pathname === '/500'
+    const isErrorPage = pathname === '/_error'
 
     const isLikeServerless =
       typeof components.Component === 'object' &&
@@ -1462,6 +1463,10 @@ export default class Server {
       res.statusCode = 404
     }
 
+    if (isErrorPage && res.statusCode === 200) {
+      res.statusCode = 404
+    }
+
     // ensure correct status is set when visiting a status page
     // directly e.g. /500
     if (STATIC_STATUS_PAGES.includes(pathname)) {

test/integration/client-navigation/test/index.test.js

@@ -25,46 +25,26 @@ describe('Client Navigation', () => {
     context.server = await launchApp(join(__dirname, '../'), context.appPort, {
       env: { __NEXT_TEST_WITH_DEVTOOL: 1 },
     })
-
-    const prerender = [
-      '/async-props',
-      '/default-head',
-      '/empty-get-initial-props',
-      '/error',
-      '/finish-response',
-      '/head',
-      '/json',
-      '/link',
-      '/stateless',
-      '/fragment-syntax',
-      '/custom-extension',
-      '/styled-jsx',
-      '/styled-jsx-external',
-      '/with-cdm',
-      '/url-prop',
-
-      '/dynamic/ssr',
-      '/dynamic/[slug]/route',
-
-      '/nav',
-      '/nav/about',
-      '/nav/on-click',
-      '/nav/querystring',
-      '/nav/self-reload',
-      '/nav/hash-changes',
-      '/nav/shallow-routing',
-      '/nav/redirect',
-      '/nav/as-path',
-      '/nav/as-path-using-router',
-
-      '/nested-cdm',
-    ]
-    await Promise.all(
-      prerender.map((route) => renderViaHTTP(context.appPort, route))
-    )
   })
   afterAll(() => killApp(context.server))
 
+  it('should not reload when visiting /_error directly', async () => {
+    const browser = await webdriver(context.appPort, '/_error')
+
+    await browser.eval('window.hello = true')
+
+    // wait on-demand-entries timeout since it can trigger
+    // reloading non-stop
+    for (let i = 0; i < 15; i++) {
+      expect(await browser.eval('window.hello')).toBe(true)
+      await waitFor(1000)
+    }
+    const html = await browser.eval('document.documentElement.innerHTML')
+
+    expect(html).toContain('This page could not be found')
+    expect(html).toContain('404')
+  })
+
   describe('with <Link/>', () => {
     it('should navigate the page', async () => {
       const browser = await webdriver(context.appPort, '/nav')