Update specs for backported event

The specs here were specific to the legacy implementation of the event
system, and therefore no longer pass on the `master` branch of Solidus
where we now use a [new implementation of the event bus](solidusio/solidus#4342).
This change attempts to refactor the tests to focus on observing the
side-effects of the object under test which should be independent of the
underlying implementation.

This test is valuable regardless of what version of Solidus we are running
against as our extension relies on the event being published when a specific
action happens and that the payload contains the necessary object.

Co-authored-by: Andrew Stewart <andrew@super.gd>

app/overrides/super_good/solidus_taxjar/spree/order_updater/fire_recalculated_event.rb

@@ -4,6 +4,10 @@ module SuperGood
   module SolidusTaxjar
     module Spree
       module OrderUpdater
+        # Responsible for introducing the `order_recalculated` event for older
+        # versions of Solidus (from 2.9 up until but not including 2.11) which
+        # do not have this event. TaxJar transaction reporting relies on this
+        # event firing when an order is recalculated.
         module FireRecalculatedEvent
           def persist_totals
             ::Spree::Event.fire 'order_recalculated', order: order

lib/super_good/engine.rb

@@ -5,6 +5,9 @@ class Engine < Rails::Engine
     isolate_namespace Spree
     engine_name 'super_good_solidus_taxjar'
 
+    # Solidus versions prior to 2.11.0 do not support auto-loading of subscribers
+    # so we need to manually register the reporting subscriber with the event
+    # bus.
     if Spree.solidus_gem_version < Gem::Version.new('2.11.0')
       require root.join('app/subscribers/super_good/solidus_taxjar/spree/reporting_subscriber')
       SuperGood::SolidusTaxjar::Spree::ReportingSubscriber.subscribe!

spec/models/spree/order_updater_spec.rb

@@ -1,12 +1,36 @@
 RSpec.describe Spree::OrderUpdater do
   describe '#update' do
+
+    module TestSubscriber
+      include Spree::Event::Subscriber
+      include SolidusSupport::LegacyEventCompat::Subscriber
+
+      event_action :order_recalculated
+
+      def order_recalculated(_event)
+      end
+    end
+
+    before do
+      if Spree::Config.use_legacy_events
+        TestSubscriber.subscribe!
+      else
+        TestSubscriber.omnes_subscriber.subscribe_to(Spree::Bus)
+      end
+    end
+
     it 'fires the order_recalculated event exactly once' do
-      stub_const('Spree::Event', class_spy(Spree::Event))
+      allow(TestSubscriber).to receive(:order_recalculated)
+
       order = create(:order)
 
       described_class.new(order).update
 
-      expect(Spree::Event).to have_received(:fire).with('order_recalculated', order: order).once
+      expect(TestSubscriber).to have_received(:order_recalculated)
+        .with(instance_of(Omnes::UnstructuredEvent))
+        .once do |event|
+          expect(event[:order]).to eq(order)
+        end
     end
   end
 end