Improve test coverage for Question::Base#permitted_next_nodes
Improve test coverage for Question::Base#permitted_next_nodes

I noticed this scenario was missing after I'd reorganised the tests into
`shoulda` contexts.

aledelcueto/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers