Allow to set Rails deprecations behavior during tests

Set the desired deprecation behavior via RAILS_DEPRECATIONS_BEHAVIOR
ENV var. Defaults to :stderr. Possible values are

  - raise - Raise ActiveSupport::DeprecationException.
  - stderr - Log all deprecation warnings to $stderr.
  - log - Log all deprecation warnings to Rails.logger.
  - notify - Use ActiveSupport::Notifications to notify deprecation.rails.
  - report - Use ActiveSupport::ErrorReporter to report deprecations.
  - silence - Do nothing

Refs: https://api.rubyonrails.org/classes/ActiveSupport/Deprecation/Behavior.html

core/lib/spree/testing_support/dummy_app.rb

@@ -80,7 +80,7 @@ class Application < ::Rails::Application
       config.action_dispatch.show_exceptions = false
     end
     config.consider_all_requests_local = true
-    config.active_support.deprecation = :stderr
+    config.active_support.deprecation = ENV['RAILS_DEPRECATIONS_BEHAVIOR'].presence&.to_sym || :stderr
     config.log_level = :debug
 
     # Improve test suite performance: