model_generator: Add support for Test::Unit

The existing install_generator supports both Test::Unit and RSpec,
but model_generator always assumes that our blueprints live in
spec/support/blueprints.rb.

This patch adds support for Test::Unit to model_generator.  Note that
there is now a small amount of code duplication between model_generator
and install_generator that should probably by factored out into a shared
module.