Scenarios testing

To be reviewed commit by commit. 

Adds tests to run through all scenarios in Smartdown questions and check flows have not been broken unexpectedly. 

Related Agile planner task: https://www.agileplannerapp.com/boards/105200/cards/6053

:exclamation: Build will not pass until the smartdown gem has been updated, new version has been published and I can bump the version in this patch. :exclamation: 
:heart: Please review https://github.com/alphagov/smartdown/pull/72 first!! :heart: