Add missing require

Commit cfa35c55bb38b27af305ad21408e181bc3cd739e removed the
buffered_logger from AS, which required the logger file. Now we need to
add the proper require here.

activesupport/lib/active_support/log_subscriber/test_helper.rb

@@ -1,4 +1,5 @@
 require 'active_support/log_subscriber'
+require 'active_support/logger'
 require 'active_support/notifications'
 
 module ActiveSupport