Test to check there are no duplicate smart answers/flows

The smart-answers part of this story: https://www.agileplannerapp.com/boards/105200/cards/6367

Bit of a strange commit since you can't automatically test a test. Feel free to pull down and temporarily create a duplicate flow if you want to see it work!
Most of the logic for getting flow names is already in the Registries, eg [`available_flows`](https://github.com/alphagov/smart-answers/blob/catch_deploy_errors/lib/smartdown_adapter/registry.rb#L70), if there's something missing (like the exclusion of `README` then that should probably live in the registries too.

Could you re-write the flow lookups to use the registry and extend those if needed?