BEIJING: For athletes at the Beijing Winter Olympics, testing positive for Covid is one of their worst nightmares, and it can begin with a knock on the door in the middle of the night. “It was insane, it was a rollercoaster,” said Kim Meylemans, a skeleton racer from Belgium. The 25-year-old posted a tearful video on Instagram last week from isolation which highlighted the perils of competing in the middle of a pandemic. Meylemans said she tested positive about a month ago, before arriving in the Chinese capital, but “here things just went from bad to worse”. “A positive test, then negative, t…