Refactor `spec/models/provider_spec.rb`
Refactor `spec/models/provider_spec.rb`

Apply better spec practices:
 - replace `should` syntax with `expect`
 - move setup to before blocks
 - use `with` and `when` contexts descriptions

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