Adds out of stock check as helper

As shared examoples, in guest_spec.rb

spec/system/consumer/checkout/guest_spec.rb

@@ -292,4 +292,30 @@
       end
     end
   end
+
+  shared_examples "when a line item is out of stock" do |session, step|
+    context "as a #{session} user" do
+      let(:user) { create(:user) }
+      before do
+        variant.on_demand = false
+        variant.on_hand = 0
+        variant.save!
+
+        if session == "logged"
+          login_as(user)
+        end
+      end
+      it "returns me to the cart with an error message" do
+        out_of_stock_check(step)
+      end
+    end
+  end
+
+  it_behaves_like "when a line item is out of stock", "guest", "#{:details}"
+  it_behaves_like "when a line item is out of stock", "guest", "#{:payment}"
+  it_behaves_like "when a line item is out of stock", "guest", "#{:summary}"
+
+  it_behaves_like "when a line item is out of stock", "logged", "#{:details}"
+  it_behaves_like "when a line item is out of stock", "logged", "#{:payment}"
+  it_behaves_like "when a line item is out of stock", "logged", "#{:summary}"
 end