[ruby/json] Setup ruby_memcheck

Hoping it might find the leak reported in ruby/json#460

ruby/json@08635312e5

Author: byroot

Diff for: test/json/test_helper.rb

@@ -1,12 +1,15 @@
 case ENV['JSON']
 when 'pure'
   $LOAD_PATH.unshift(File.expand_path('../../../lib', __FILE__))
+  $stderr.puts("Testing JSON::Pure")
   require 'json/pure'
 when 'ext'
+  $stderr.puts("Testing JSON::Ext")
   $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
   require 'json/ext'
 else
   $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
+  $stderr.puts("Testing JSON")
   require 'json'
 end