Do not clear subscribers when resetting preferences

Subscribers are loaded once at runtime during the app initialization
process, so if they get reset by `reset_spree_preferences` then the
original value will not come back.

core/lib/spree/testing_support/preferences.rb

@@ -14,7 +14,9 @@ module Preferences
       #
       # @deprecated
       def reset_spree_preferences(&config_block)
-        Spree::Config.instance_variables.each { |iv| Spree::Config.remove_instance_variable(iv) }
+        Spree::Config.instance_variables.
+          reject { |iv| iv == :@events_configuration }.
+          each { |iv| Spree::Config.remove_instance_variable(iv) }
         Spree::Config.preference_store = Spree::Config.default_preferences
 
         if defined?(Railties)