Add vtree tests for server rendering example

This adds tests against the vtree structure for the *server-rendering* example. These are UI tests that can run on the server. This also shows how to use *[vtree-select][]* to make traversing a vtree a bit easier. 

It's possible to write a higher order selector module that can select from both a vtree and a real dom and return the same objects so you could write tests that run both on the server and against an actual dom.

[vtree-select]: https://www.npmjs.org/package/vtree-structure
preference for `assert.equal(spanText, state.description)`