Ballast tanks in testplanes:

Water-filled tanks simulate passenger weight for different takeoff and landing displacement weights

Take a Peek Inside the 747-8 Test Plane: 

https://www.wired.com/2010/02/peek-inside-boeing-747-8/

First look: Inside Boeing's first prototype 777X:

https://thepointsguy.co.uk/news/boeing-777x-first-look-dubai-airshow/