The 777 was developed with an "iron bird". A mockup of all the aircraft systems where compatibility issues could be researched.
Maybe the "Iron Bird" term has been expanded. On the 707 program, and maybe through the 767 program, it referred to the structural test fixture that held an airframe primary structure in place, and used hydraulic actuators to flex wings and other key elements to simulate flight operation cycles with increasing severity to determine limits where structural failures resulted. Iron Bird was an appropriate description because the fixture was made of very heavy structural steel, but the airframe was a standard production configuration. Access to the facility was controlled because as limits were reached parts could, and did, fly off the airframe under test when they reached the braking point. Systems tests were run in separate facilities.