The new terminal is supposed to open on 22/23 April so they are doing test runs now with fake passengers.
The video is in German, but you can at least get a glimpse of it. Because this has been in planning for 2 years, the tickets that many people had, said they were going to Dubai. Pretty sure the airport thought that was a safe destination to plan on.
They will do another test run with more airlines and more passengers soon.
https://www.hessenschau.de/tv-sendung/terminal-3-im-testbetrieb,container-beitrag-38836.html