Improve the flakey determine_target_shipment spec

- Explicitly set the count on hand
- Add pre-flight expectations
- Aggregate failures

The flakeyness is very hard to reproduce and if the supposed fixes
won't have an effect at least this should give use more information
on why it failed.

core/spec/models/spree/order_inventory_spec.rb

@@ -171,14 +171,21 @@
         end
 
         context 'when there is not enough availability at any stock location' do
+          before { stock_item.set_count_on_hand 0 }
+
           it 'falls-back selecting first non-shipped shipment that leaves from same stock_location' do
-            shipment = subject.send(:determine_target_shipment, 1)
+            required_quantity = 1
+            shipment = subject.send(:determine_target_shipment, required_quantity)
             shipment.reload
 
-            expect(shipment.shipped?).to be false
-            expect(shipment.inventory_units_for(variant)).to be_empty
-            expect(variant.stock_location_ids.include?(shipment.stock_location_id)).to be true
-            expect(shipment.stock_location).not_to eql stock_item.stock_location
+            aggregate_failures do
+              expect(stock_item.count_on_hand).to eq(0)
+              expect(stock_item.backorderable?).to eq(false)
+              expect(shipment.shipped?).to be false
+              expect(shipment.inventory_units_for(variant)).to be_empty
+              expect(variant.stock_location_ids.include?(shipment.stock_location_id)).to be true
+              expect(shipment.stock_location).not_to eql stock_item.stock_location
+            end
           end
         end
       end