NASHVILLE, Tenn.—Two-time champion Alysa Liu withdrew from the U.S. Figure Skating Championships after testing positive for COVID-19 on Friday, less than a day after a third-place finish in the short program put her in position to make the Olympic team. The 16-year-old Liu is at least the fifth competitor to test positive for the virus in the run-up and during nationals, and the third that was expected to make the team for the Beijing Games. The pairs tandem of Alexa Knierim and Brandon Frazier dropped out the day before the start of competition when he returned a positive test. Just like Knierim and Frazier, who plan to petition for one of the two pairs spots on the Olympic team, Liu has informed U.S. Figure Skating she will petition for one of the three women’s spots based on her body of recent work. “I’m fully vaccinated, have been wearing a n95 …