By Amruta Khandekar and Noel Randewich (Reuters) – The S&P 500 tumbled on Friday, putting the widely followed benchmark on the verge of confirming it has been in a bear market since hitting a record high in January. Worries about surging inflation and rising interest rates have pummeled the U.S. stock market this year, with danger signals from Walmart Inc and other retailers this week adding to fears about the economy. Wall Street opened stronger on Friday morning before turning sharply negative and adding to deep losses sustained earlier in the week. “Any positivity is being sold in a very he…