The University of Virginia’s men’s basketball team, the 2019 national champion, pulled out of the Atlantic Coast Conference tournament on Friday after someone inside the program tested positive for the coronavirus. Virginia’s withdrawal was the second in 24 hours for the conference tournament — Duke dropped out on Thursday — because of the virus. And it immediately put into some doubt whether Virginia (18-6) will be able to compete in the N.C.A.A. tournament, which is scheduled to begin next week.