Fix flakey test related with non unique location generation
Fix flakey test related with non unique location generation

Tests like the following:
https://github.com/ministryofjustice/Claim-for-Crown-Court-Defence/blob/0ea00787062887099785c2c2ea6ec307d34ef53a/spec/controllers/external_users/claims_controller_spec.rb#L657

were creating factory data for a location using Faker::Address.city but
even though the location model expects unique names, the existent
definition (using Faker) did not guarantee that.

This fixes makes use of the `unique` method in Faker:
https://github.com/stympy/faker#ensuring-unique-values

ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments