Can't always login with persistent users

Because the persistent user steps skip existing users if they're already created, they don't get registered in the hash of users we can login with. Thus, when running a test which a) creates persistent users and b) tries to And I login as "persistent-user" will fail the second time it runs, because the user is already in the system.

I believe we can fix this by simply registering duplicate users in the hash we use to login with, but not try to create an actual Drupal user (perhaps reset the password to the one from the current step?)