[WIP] Adds tests for build pipeline

The purpose of this PR is to test, refactor the build pipeline and decrease the size of `Brocfile.js`.
I wanted to keep things isolated (`mocha` dependency has nothing to do with `ember` itself) hence a separate `package.json` file.
Maybe `build` folder should be a separate repo and `npm` package that we use in `Brocfile.js`.

- [x] extract `vendoredPackage`
- [x] extract `htmlbarsPackage`
- [ ] extract `concatES6`
- [ ] extract `es6Package`
- [ ] extract `packageDependencyTree`
- [ ] extract `vendoredEs6Package`
- [ ] extract `buildRuntimeTree`
- [ ] extract `defeatureify` ?
- [ ] use `broccoli-funnel`