Use automated exercise/testing

@NelleV pointed out that it's very useful for students to get automated feedback on their exercises — we should find a way (probably pre-existing e.g. nbgrader) to make exercise solutions testable automatically.

该 issue 未指定文件或测试；请先检查练习材料，并评估建议的 nbgrader 方案。完成的标准是选择并记录一条可针对练习解答运行的自动化反馈和测试路径。

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