Load defaults for the latest Rails minor version in the dummy app

Before this change, the dummy app used in the test suite was not loading
any new default that Rails has added since 5.0.

However, we still use old defaults for three settings that make our test
suite fail. Each one of them will be worked out in individual commits.

See https://api.rubyonrails.org/classes/Rails/Application/Configuration.html#method-i-load_defaults

core/lib/spree/testing_support/dummy_app.rb

@@ -46,6 +46,12 @@ def self.setup(gem_root:, lib_name:, auto_migrate: true)
   end
 
   class Application < ::Rails::Application
+    config.load_defaults("#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}")
+    # TODO: Remove the three following settings once the codebase has been adapted
+    # to load_defaults
+    config.active_record.has_many_inversing = nil
+    config.active_storage.track_variants = nil
+    config.active_record.belongs_to_required_by_default = false
     config.eager_load = false
     config.cache_classes = true
     config.cache_store = :memory_store

core/spec/lib/spree/core/testing_support/dummy_app_spec.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe DummyApp do
+  it 'loads default from the Rails version in use' do
+    expect(
+      DummyApp::Application.config.loaded_config_version
+    ).to eq("#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}")
+  end
+end
+