Move test app into spec/dummy

This isn't library code, it's test code. It doesn't belong in `lib`. Common
convention in engines is to have the dummy app in `spec/dummy`.
If you take a look at the `Rakefile` you'll see that I load the dummy app's rake tasks into a namespace. We used to load them at the root, which made `rake -T` output confusing. I think prefixing it with a `dummy` namespace makes it clear you're performing a rake task on the dummy app.