trivial: use require_relative

Author: enebo

test/db/mysql/rails_test.rb

@@ -1,6 +1,6 @@
 # -*- encoding : utf-8 -*-
 
-require File.expand_path('test_helper', File.dirname(__FILE__))
+require_relative 'test_helper'
 
 class MySQLRailsTest < Test::Unit::TestCase