Schematics testing infrastructure: compare Trees, copy Trees

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Desired functionality

When unit testing a schematic or a rule, I have sometimes found that it would be convenient if I could take two trees and compare them. Or at least ask whether the two trees are equivalent.

As part of this, it would also be convenient if there was some Tree-copying function - specifically for checking that a Tree has _not_ changed after some Rule call, but possibly for other cases as well.

首先检查 Schematics 的测试基础设施，以及 issue 中引用的 Tree 和 Rule API。确定树的等价性和复制应如何工作，然后添加覆盖，证明等价的树可以成功进行比较，并且可以在调用 rule 后检查复制的树是否发生更改。

由索引模型根据 Issue 内容生成。