| Module | ActionController::TestCase::RaiseActionExceptions | 
| In: | lib/action_controller/test_case.rb | 
When the request.remote_addr remains the default for testing, which is 0.0.0.0, the exception is simply raised inline (bystepping the regular exception handling from rescue_action). If the request.remote_addr is anything else, the regular rescue_action process takes place. This means you can test your rescue_action code by setting remote_addr to something else than 0.0.0.0.
The exception is stored in the exception accessor for further inspection.