LOS ANGELES — Testing has ruled out beef patties as the source of the outbreak of E. coli poisoning tied to McDonald’s Quarter Pounders, the company said Sunday. It will resume selling the Quarter Pounder in the coming week. The U.S. Food and Drug Administration continues to believe that slivered onions from a single supplier are the likely source of contamination, McDonald’s said. As of Friday, the outbreak had expanded to at least 75 people sick in 13 states, federal health officials said.