By Jeff Mason WASHINGTON (Reuters) – President Joe Biden has tested negative for COVID-19 after testing positive with a breakthrough case for days, the White House physician said on Saturday. The Democratic president, “in an abundance of caution,” will remain in isolation until he tests negative on a second test, Dr. Kevin O’Connor said in a memo released by the White House. Biden, 79, emerged from isolation at the White House on July 27 after testing positive for COVID-19 for the first time on July 21. He tested positive again on July 30 in what O’Connor described as a “rebound” case seen in …