conference: refactor code and tests

spec/models/conference_spec.rb

@@ -1588,7 +1588,7 @@ describe Conference do
     let!(:conference) { create(:conference) }
 
     it 'creates free tickets' do
-      free_ticket = Ticket.find_by(conference: conference)
+      free_ticket = conference.tickets.first
       expect(free_ticket).not_to be_nil
       expect(free_ticket.price_cents).to eq(0)
     end