Mount warden in tests, too.

Adding warden to config.ru does configure Warden for when using `rackup`. But tests don't run `config.ru`. A simple solution is to include warden in the application, which I have done by creating a Rack app with `Rack::Builder`:

``` ruby
module Rackup
  class << self
    def app
      @app ||= Rack::Builder.new do
        use Rack::Session::Cookie
        Warden::Manager.serialize_into_session { |user| user.id }
        Warden::Manager.serialize_from_session { |id| User.get(id) }
        use Warden::Manager do |manager|
          manager.default_strategies :password, :basic
          manager.failure_app = Acme::BadAuthentication
        end
        run Acme::App.new
      end
    end
  end
end
```

The `config.ru` needs to use this app, and so do tests.