binding ruby: Fix API for test

Before: User must manage PID of milter.
    class TestSomeMilter < Test::Unit::TestCase
      def setup
        @pid = spawn_milter(milter_path, *args)
      end
      def teardown
        kill_milter(@pid)
      end
    end

After: User don't need to manager PID of milter.
    class TestSomeMilter < Test::Unit::TestCase
      def setup
        @milter_runner = Milter::Client::Test::MilterRunner.new(milter_path, *args)
        @milter_runner.run
      end
      def teardown
        @milter_runner.stop
      end
    end