LONDON—The new potentially more contagious Omicron variant of the coronavirus popped up in more European countries on Saturday, just days after being identified in South Africa, leaving governments around the world scrambling to stop the spread. The UK on Saturday tightened its rules on mask-wearing and on testing of international arrivals after finding two cases. New cases were confirmed Saturday in Germany and Italy, with Belgium, Israel, and Hong Kong also reporting that the variant has been found in travelers. In the United States, Dr. Anthony Fauci, the government’s top infectious diseases expert, said he would not be surprised if the Omicron variant was already in the United States, too. “We have not detected it yet, but when you have a virus that is showing this degree of transmissibility … it almost invariably is ultimately going to go essentially all over,” Fauci said on NBC television. Because of fears that …