WASHINGTON — In highly unusual testimony inside the federal supermax prison, a former operative for Al Qaeda has described prominent members of Saudi Arabia’s royal family as major donors to the terrorist network in the late 1990s and claimed that he discussed a plan to shoot down Air Force One with a Stinger missile with a staff member at the Saudi Embassy in Washington.... There has long been evidence that wealthy Saudis provided support for bin Laden, the son of a Saudi construction magnate, and Al Qaeda before the 2001 attacks.