test: pipe some error output if npm fails

This test now prints out some child error output if the npm child proc
fails, allowing us to debug easier.

Refs: nodejs#12480

test/parallel/test-npm-install.js

@@ -6,7 +6,7 @@ if (!common.hasCrypto) {
 }
 
 const path = require('path');
-const spawn = require('child_process').spawn;
+const exec = require('child_process').exec;
 const assert = require('assert');
 const fs = require('fs');
 
@@ -26,11 +26,6 @@ const npmPath = path.join(
   'npm-cli.js'
 );
 
-const args = [
-  npmPath,
-  'install'
-];
-
 const pkgContent = JSON.stringify({
   dependencies: {
     'package-name': common.fixturesDir + '/packages/main'
@@ -47,17 +42,22 @@ env['NPM_CONFIG_PREFIX'] = path.join(npmSandbox, 'npm-prefix');
 env['NPM_CONFIG_TMP'] = path.join(npmSandbox, 'npm-tmp');
 env['HOME'] = path.join(npmSandbox, 'home');
 
-const proc = spawn(process.execPath, args, {
+exec(`${process.execPath} ${npmPath} install`, {
   cwd: installDir,
   env: env
-});
+}, common.mustCall(handleExit));
+
+function handleExit(error, stdout, stderr) {
+  const code = error ? error.code : 0;
+  const signalCode = error ? error.signal : null;
+
+  if (code !== 0) {
+    process.stderr.write(stderr);
+  }
 
-function handleExit(code, signalCode) {
   assert.strictEqual(code, 0, `npm install got error code ${code}`);
   assert.strictEqual(signalCode, null, `unexpected signal: ${signalCode}`);
   assert.doesNotThrow(function() {
     fs.accessSync(installDir + '/node_modules/package-name');
   });
 }
-
-proc.on('exit', common.mustCall(handleExit));