Schematics testing infrastructure: ability to easily bootstrap a Tree

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

### Desired functionality

I'd like to be able to more easily unit test individual Rules. and to do this, I'd like to be able to easily create a Tree on the fly.

My current strategy is to create a new schematic for the sole purpose of testing. But there is quite a bit of boilerplate that goes into this: the schematic code, the schema, the build rules, the awkward dependency of the test on this schematic, and the construction of a new schematicRunner in the test. Furthermore, different Rules need different Tree "boilerplate" for testing, so instead of creating a new schematic for each, I've just been jamming all the different types of files I need into this one test-only schematic, which is becoming unwieldy - which files are for testing which Rules?

Sometimes, I am testing a really simple Rule that only operates on one file, and I just want to be able to ask some function: "give me a tree with one file at this certain path with this certain content" or "give me a tree with an empty file at this path" and so on.

I think it'd be great if there was some testing infrastructure that could cook up a quick Tree in place. Or - does something like this already exist?

首先閱讀現有的僅用於測試的 Schematic 設定，包括其 Schematic 程式碼、schema、建置規則以及測試中 SchematicRunner 的建構。比較個別 Rules 使用的不同 Tree 樣板，並定義一個能夠建立所要求檔案和內容的小型測試入口點；當 Rules 不需要專用 Schematic 即可進行單元測試時，即表示完成。

由索引模型根據 Issue 內容生成。