update specs to reflect changes to the model

spec/services/candidates/registrations/subject_and_date_information_spec.rb

@@ -105,14 +105,11 @@
         subject.bookings_subject_id = 2
       end
 
-      before { allow(Bookings::PlacementDateSubject).to receive(:find_by).and_return('a') }
+      before { allow(subject).to receive(:find_placement_date_subject).and_return('a') }
 
       specify 'should find the placement date via its id' do
         subject.placement_date_subject
-        expect(Bookings::PlacementDateSubject).to have_received(:find_by).with(
-          bookings_placement_date_id: 1,
-          bookings_subject_id: 2
-        )
+        expect(subject).to have_received(:find_placement_date_subject)
       end
     end