add fact service_provider during unit tests

During unit tests with puppetlabs-postgresql module we are getting follwoing error during unit tests :

       error during compilation: Evaluation Error: Error while evaluating a Function Call, pick(): must receive at least one non empty value (file: /builds/directmairie/puppet-directmairie/spec/fixtures/modules/postgresql/manifests/params.pp, line: 172, column: 10) on node runner-uux1cxd-project-497-concurrent-0.adullact.lan

At line 172 in postgresql/manifests/params.pp we have if pick($service_provider, $facts['service_provider']) == 'systemd' {

The error message is understood as an non existing fact service_provider during unit tests. The puppetlabs-postgresql use this : https://github.com/puppetlabs/puppetlabs-postgresql/commit/423dcbf30da6b0520134b3a095fb3906eda64035