When the request.remote_addr remains the default for testing, which is 0.0.0.0, the exception is simply raised inline (skipping 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.

Methods
I
R
Class Public methods
included(base)

Source: show | on GitHub 

# File actionpack/lib/action_controller/test_case.rb, line 530
def self.included(base)
  unless base.method_defined?(:exception) && base.method_defined?(:exception=)
    base.class_eval do
      attr_accessor :exception
      protected :exception, :exception=
    end
  end
end
Instance Protected methods
rescue_action_without_handler(e)

Source: show | on GitHub 

# File actionpack/lib/action_controller/test_case.rb, line 540
def rescue_action_without_handler(e)
  self.exception = e

  if request.remote_addr == "0.0.0.0"
    raise(e)
  else
    super(e)
  end
end