Using the ruby debugger in tests

Is there any way to attach the ruby debugger to a functional test? I was hoping that something like :
ruby -r debug test/functional/hot_seat_controller_test.rb
would work, but it dies with the following trace:

./script/../config/boot.rb:14: `uninitialized constant Rails' (NameError)
         from /usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:21:in `require'
         from script/runner:3
./script/../config/boot.rb:14:unless defined?(Rails::Initializer)

Any ideas?
Jon

$ gem install ruby-debug
$ rdebug test/functional/hot_seat

_controller_test.rb