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.

まず、Schematics のテストインフラストラクチャと、issue で参照されている Tree および Rule API を調べます。Tree の同等性とコピーがどのように動作すべきかを確認し、その後、同等の Tree が正常に比較されること、および Rule の呼び出し後にコピーされた Tree の変更を確認できることを示すカバレッジを追加します。

索引モデルが issue の本文から書いたものです。