As Joe Biden's verbal gaffes, shaky voice and other troubling signs have brought an intense focus on the US president's mental acuity, health experts are calling on him and rival Donald Trump to pass additional cognitive tests, even while warning against leaping to conclusions.Such tests, experts said, could either help repudiate speculation that the 81-year-old president's mental state is in worrying decline -- or else confirm it -- and could enlighten voters on the mental abilities of Trump, w