The Green Hill Fire Station behind Town Hall remains closed while a black substance found there is tested to see if it’s mold, according to Town Manager Philip Lemnios. He said results probably will come back early this week. In the meantime, he said, the fire engine usually housed there has been moved to the Central Fire Station, 2.6 miles away at the corner of A Street and Nantasket Avenue. Lemnios said the move will add about three minutes to response time to some areas of town. “We’re always concerned about that, but we have no other option at this point,” he said.