Run tests with latest liquid/c gem

test/test_helper.rb

@@ -15,6 +15,11 @@ if env_mode = ENV['LIQUID_PARSER_MODE']
 end
 Liquid::Template.error_mode = mode
 
+if ENV['LIQUID-C'] == '1'
+  puts "-- LIQUID C"
+  require 'liquid/c'
+end
+
 if Minitest.const_defined?('Test')
   # We're on Minitest 5+. Nothing to do here.
 else