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.

Comienza examinando la infraestructura de pruebas de Schematics y las API de Tree y Rule referenciadas en el issue. Determina cómo deben comportarse la equivalencia y la copia de árboles, y después añade cobertura que demuestre que los árboles equivalentes se comparan correctamente y que se puede comprobar si un árbol copiado ha cambiado después de una llamada a una regla.

Escrito por el modelo de indexación a partir del texto del issue.