FIX: Multisite DB was leaving old data in test mode

This commit introduces a new helper to enable transactional fixtures
when testing multisite. This would show up as tests that passed the
first time then failed the second time due to stale data being leftover.

spec/multisite/distributed_cache_spec.rb

@@ -1,8 +1,6 @@
 require 'rails_helper'
 
 RSpec.describe 'Multisite SiteSettings', type: :multisite do
-  let(:conn) { RailsMultisite::ConnectionManagement }
-
   def cache(name, namespace: true)
     DistributedCache.new(name, namespace: namespace)
   end
@@ -15,7 +13,7 @@ RSpec.describe 'Multisite SiteSettings', type: :multisite do
 
       expect(cache1.hash).to eq('default' => true)
 
-      conn.with_connection('second') do
+      test_multisite_connection('second') do
         message = MessageBus.track_publish(DistributedCache::Manager::CHANNEL_NAME) do
           cache1['second'] = true
         end.first