PHPUnit:模拟不存在的类

是否可以在PHPUnit中创建一个不存在的类的模拟？
假设我有一些类创建另一个类的实例,例如:

class TaskRunner
{
    public function runTasks()
    {
        // Run through some loop to get tasks and store each in $taskName
        // Get task instance by given task name 
        $task = $this->getTaskInstance($taskName);
        if ($task instanceof AbstractTask) {
            $task->run();
        }
    }

    protected function getTaskInstance($taskName)
    {
        // Just an example
        return new $taskName();
    }
}