Tests to ensure rendering dates in templates is working correctly.
Actually, I was running into invalid templates not giving warnings when
I was trying to render a date.
Provide unit test support RenderThing.
One observation is that creating the site.Tmpl variable is a one time
event. site.Tmpl doesn't like additional templates with the same name.
This means that updating a template while in --watch mode requires
throwing away the entire Site object and creating a new one. Not that
this is a bad idea, but it is something I discovered while working on
these unit tests.
