add test for france connect service

spec/services/france_connect_service_spec.rb

@@ -4,10 +4,20 @@ describe FranceConnectService do
 
   describe '.retrieve_user_informations' do
     let(:code) { 'plop' }
-
+    let(:mocky) { 'my mocky' }
+    let(:user_info) { 'user_informations' }
+    subject { described_class.retrieve_user_informations code }
+    before do
+      allow_any_instance_of(FranceConnectClient).to receive(:access_token!).and_return(mocky)
+      allow(mocky).to receive(:userinfo!).and_return(user_info)
+    end
     it 'set code for FranceConnectClient' do
       expect_any_instance_of(FranceConnectClient).to receive(:initialize).with(code: code)
       described_class.retrieve_user_informations code
     end
+
+    it 'returns user informations' do
+      expect(subject).to eq(user_info)
+    end
   end
 end