Update concurrency spec after refactor

In order to make the spec fail if the controller was not thread safe, it
uses breakpoints. One of those breakpoints was set for a method that has
now been removed.

I changed the method that is used for the breakpoint and changed `allow`
to `expect` so that this spec will fail if we remove that method as
well. Future version of Rspec will check if a mocked method actually
exists but our version just mocks it anyway. This is one way how specs
can become invalid after refactoring.

spec/controllers/checkout_controller_concurrency_spec.rb

@@ -30,8 +30,11 @@ describe CheckoutController, concurrency: true, type: :controller do
     # New threads start running straight away. The breakpoint is after loading
     # the order and before advancing the order's state and making payments.
     breakpoint.lock
-    allow(controller).to receive(:check_order_for_phantom_fees) do
+    expect(controller).to receive(:fire_event).with("spree.checkout.update") do
       breakpoint.synchronize {}
+      # This is what fire_event does.
+      # I did not find out how to call the original code otherwise.
+      ActiveSupport::Notifications.instrument("spree.checkout.update")
     end
   end