The custom build test now runs in production mode (#6818)

I also removed the copy it had of `runNextCommand`

test/integration/production-build-dir/test/index.test.js

 /* eslint-env jest */
 /* global jasmine */
-import { dirname, join } from 'path'
+import { join } from 'path'
 import {
   nextServer,
+  runNextCommand,
   startApp,
   stopApp,
   renderViaHTTP
 } from 'next-test-utils'
-import spawn from 'cross-spawn'
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 5
 
-function runNextCommand (argv, options = {}) {
-  return new Promise((resolve, reject) => {
-    console.log(`Running command "next ${argv.join(' ')}"`)
-    const instance = spawn('node', [join(dirname(require.resolve('next/package')), 'dist/bin/next'), ...argv], { ...options.spawnOptions, cwd: join(__dirname, '..'), stdio: ['ignore', 'pipe', 'pipe'] })
-
-    let stderrOutput = ''
-    if (options.stderr) {
-      instance.stderr.on('data', function (chunk) {
-        stderrOutput += chunk
-      })
-    }
-
-    let stdoutOutput = ''
-    if (options.stdout) {
-      instance.stdout.on('data', function (chunk) {
-        stdoutOutput += chunk
-      })
-    }
-
-    instance.on('close', () => {
-      resolve({
-        stdout: stdoutOutput,
-        stderr: stderrOutput
-      })
-    })
-
-    instance.on('error', (err) => {
-      err.stdout = stdoutOutput
-      err.stderr = stderrOutput
-      reject(err)
-    })
-  })
-}
-
 describe('Production Custom Build Directory', () => {
   describe('With basic usage', () => {
     it('should render the page', async () => {
-      const result = await runNextCommand(['build', 'build'], { stdout: true, stderr: true })
+      const result = await runNextCommand(['build', 'build'], {
+        cwd: join(__dirname, '..'),
+        spawnOptions: { env: { ...process.env, NODE_ENV: 'production' } },
+        stdout: true,
+        stderr: true
+      })
       expect(result.stderr).toBe('')
 
       const app = nextServer({

test/lib/next-test-utils.js

@@ -71,10 +71,12 @@ export function findPort () {
 }
 
 export function runNextCommand (argv, options = {}) {
-  const cwd = path.dirname(require.resolve('next/package'))
+  const nextDir = path.dirname(require.resolve('next/package'))
+  const nextBin = path.join(nextDir, 'dist/bin/next')
+  const cwd = options.cwd || nextDir
   return new Promise((resolve, reject) => {
     console.log(`Running command "next ${argv.join(' ')}"`)
-    const instance = spawn('node', ['dist/bin/next', ...argv], { ...options.spawnOptions, cwd, stdio: ['ignore', 'pipe', 'pipe'] })
+    const instance = spawn('node', [nextBin, ...argv], { ...options.spawnOptions, cwd, stdio: ['ignore', 'pipe', 'pipe'] })
 
     if (typeof options.instance === 'function') {
       options.instance(instance)