[ruby/json] Enable coverage before loading simplecov

Fix: ruby/json#853

Simplecov end up requiring json so we need to start collecting
coverage before.

ruby/json@ca72019fd3

Author: byroot

test/json/test_helper.rb

@@ -1,5 +1,8 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
+require 'coverage'
+Coverage.start
+
 begin
   require 'simplecov'
 rescue LoadError