code: function checkExec (mode, callback) {
    if (!exercise.passed)
      return callback(null, true) // shortcut if we've already had a failure
  
    execWith(copyTempDir, 'testing', function (err, pass) {
      if (err)
        return callback(err)
      if (!pass) {
        exercise.emit('fail', 'JavaScript code loads addon, invokes `length(str)` method and prints the return value')
        return callback(null, false)
      }
  
      execWith(copyTempDir, 'this is a longer test string, with spaces in it', function (err, pass) {
        if (err)
          return callback(err)
  
        exercise.emit(pass ? 'pass' : 'fail', 'Add-on receives string, calculates length and returns value')
        callback(null, pass)
      })
    })
  }
docs: run a full execution of their code & addon, uses a `require()` in a child process and check the stdout for expected