Update tests to use 'a' instead of 'per'
Update tests to use 'a' instead of 'per'

alphagov/digitalmarketplace-content-loader#109 replaces 'per' with 'a',
we need to update our tests to reflect this.

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend