Updated factories loading after recent changes in solidus

This commit reflects changes made by Solidus [PR #169]
(solidusio/solidus_dev_support#169 (comment)) When testing extensions
using Solidus vesions that support the definition_file_paths,
it will load the core factory first and then the ones defined in
`lib/extension_name/testing_support/factories`.
Additionally, this extention makes use of assertions
(i.e. `assert_nothing_raised) found in the version of Rails not used by
Solidus 2.10, therefore it must be manually configured in `Rspec.config`.

lib/solidus_expedited_exchanges/testing_support/factories.rb

@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+end

spec/spec_helper.rb

@@ -16,8 +16,8 @@
 # in spec/support/ and its subdirectories.
 Dir[File.join(File.dirname(__FILE__), 'support/**/*.rb')].each { |f| require f }
 
-# Requires factories defined in lib/solidus_expedited_exchanges/factories.rb
-require 'solidus_expedited_exchanges/factories'
+# Requires factories defined in solidus_dev_support
+SolidusDevSupport::TestingSupport::Factories.load_for(SolidusExpeditedExchanges::Engine)
 
 RSpec.configure do |config|
   config.include(ActiveSupport::Testing::Assertions)