Add test for bad flash message appearing on account creation.
Add test for bad flash message appearing on account creation.

https://trello.com/c/4JthSqdx/614-flash-message-problem-on-buyer-account-creation

alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend