Steven Naismith knows how difficult the task is for his Hearts side as they prepare to visit Celtic Park on Saturday. Hearts have only picked up a single point in their opening four Scottish Premiership fixtures, at home to Rangers on the opening day. Since then they’ve suffered six consecutive defeats in all competitions at home and abroad, unable to score enough goals or effectively shut out a wide range of opponents. It doesn’t bode well for them now visiting Glasgow for what Naismith believes will be their ‘toughest’ test to date. Hearts boss Steven Naismith talks up Celtic’s qualityAlthou…