Add a test to render a <template>'s contents

I'm working on a project where we integrate web components into a web application using the preact framework. For the most part, the issues this site have raised have proven applicable and been very helpful in steering the way we design the application.

One aspect that took me by surprise, though, is that there may be an issue when a custom element uses template tags in the light children area. In particular, I encountered issues with the Vaadin Grid, which uses inner templates to define the columns. Something seemed to go wrong in the way the JSX/h engine generated these teplate tags.

It's a little hard to explain what I was seeing. It was like the template tags weren't quite inert, and the inside of the template turned into strings. Or something. I do think there's an issue here.