Now I wonder how the test it ? Is it on a software emulator on modern equipment or do they have a Voyager replica ?
To me that souns like we should test all revisions of the CPU, not none of them
They have a simulator for one of the three computers on Voyager, the Computer Command Subsystem. They don't have a simulator for the other two computers, the Attitude and Articulation Control Subsystem and the Flight Data Subsystem. They used to have a Voyager replica, the Capability Demonstration Lab, but they got rid of it after they moved the Voyager team into a satellite office following the end of the main mission. There's more information here if you're interested: https://arc.aiaa.org/doi/pdf/10.2514/6.2016-2415
Simulators for the spacecraft and replicas for some bits of hardware.