HAVANA (Reuters) - A celebration this week of ailing Cuban leader Fidel Castro's 80th birthday has turned into a high-stakes test of his health and political future as the world watches to see if he is well enough to attend. After four months out of public view, the events starting on Tuesday may tell much about whether Castro is recovering from an undisclosed illness and can again govern the communist country he has led for nearly half a century. Castro has been seen only in photos and videos since shortly before announcing July 31 he had intestinal surgery and temporarily...