Extract shared example for later reuse

These specs don't really require a specific first level scenario,
so we're first extracting them to a shared example, and with the
following commit we're going to use it in other existing scenarios.

Author: spaghetticode

core/spec/models/spree/fulfilment_changer_spec.rb

@@ -81,45 +81,47 @@
     end
   end
 
-  subject { shipment_splitter.run! }
-
-  before do
-    variant.stock_items.first.update_column(:count_on_hand, 100)
-  end
-
-  context "when the current shipment stock location is the same of the target shipment" do
-    let(:current_shipment_inventory_unit_count) { 1 }
-    let(:quantity) { current_shipment_inventory_unit_count }
-
+  shared_examples_for "properly manages inventory units" do
     context "when the stock location is empty" do
-      before do
-        variant.stock_items.first.update_column(:count_on_hand, 0)
-      end
+      let(:stock_item) { variant.stock_items.find_by!(stock_location: current_shipment.stock_location) }
+
+      before { stock_item.update_column(:count_on_hand, 0) }
 
-      context "when the inventory unit is backordered" do
+      context "when there are backordered inventory units" do
         before do
-          current_shipment.inventory_units.first.update state: :backordered
+          current_shipment.inventory_units.first.update(state: :backordered)
         end
 
-        it "creates a new backordered inventory unit" do
-          subject
-          expect(desired_shipment.inventory_units.first).to be_backordered
+        it "doesn't change the order inventory units state" do
+          expect { subject }.not_to change { order.inventory_units.map(&:state).sort }
         end
       end
 
-      context "when the inventory unit is on hand" do
+      context "when all inventory units are on hand" do
         before do
-          current_shipment.inventory_units.first.update state: :on_hand
+          current_shipment.inventory_units.update_all(state: :on_hand)
         end
 
-        it "creates a new on hand inventory unit" do
-          subject
-          expect(desired_shipment.inventory_units.first).to be_on_hand
+        it "doesn't change the order inventory units state" do
+          expect { subject }.not_to change { order.inventory_units.map(&:state).sort }
         end
       end
     end
   end
 
+  subject { shipment_splitter.run! }
+
+  before do
+    variant.stock_items.first.update_column(:count_on_hand, 100)
+  end
+
+  context "when the current shipment stock location is the same of the target shipment" do
+    let(:current_shipment_inventory_unit_count) { 1 }
+    let(:quantity) { current_shipment_inventory_unit_count }
+
+    it_behaves_like "properly manages inventory units"
+  end
+
   context "when tracking inventory is not set (same as false)" do
     let(:current_shipment_inventory_unit_count) { 2 }
     let(:quantity) { 1 }