Fixes Bug 1032030, hint errors in build/test/integration/build.test.js r=yurenju

line 148 ~ 162, command can be declared by `var cmd = '....';` and use it in `helper.exec()`
```js
    var cmd = 'NODE_MODULES_GIT_URL=' +
      'https://git.mozilla.org/b2g/gaia-node-modules.git make node_modules';
    helper.exec(cmd, function(error, stdout, stderr) {
      helper.checkError(error, stdout, stderr);

      var modulesTarPath = path.join(process.cwd(), 'git-gaia-node-modules',
        '.git');
      assert.ok(fs.existsSync(modulesTarPath));

      var packageJson = path.join(process.cwd(), 'node_modules',
        'marionette-client', 'package.json');
      assert.ok(fs.existsSync(packageJson));

      done();
    });
```