Is it possible to debug unit tests in Ruby on Rails?
Yes, it is possible.
First of all you must install ruby-debug.
sudo gem install ruby-debug
then you write
debugger
immediately before the line you wish to start debugging.
Next step: run your unit tests as you would do normaly.
At the moment that ruby reaches the line that contain the debugger directive it will stop and show you a console prompt.
Now you can debug in the same way as you would do with gdb.
This link explains it in a very nice way: http://blog.nanorails.com/articles/2006/07/14/a-better-rails-debugger-ruby-debug
I hope this informtaion will be useful to you.
As a (late) update, you could also use byebug which is targeting Ruby 2 debugging.
It's also included by default in the Gemfile that comes with any new Rails project and there is also a nice tutorial in the Debugging Rails Applications Guide.
You can just insert
byebuganywhere in your test file and then run the test as you normally would.