Way back in the early days of TCAS, most of the airborne certification tests were flown against false targets - 'a hacked system' - external to the test aircraft in flight. I decline to explain how in an open forum, but very simple.
Even if a spurious flight deviation generated a hazardous real aircraft conflict, then the TACAS logic has further protections.
Similar tests for EGPWS, but it was much more fun flying low level in Wales on a clear day - planning to miss the mountain tops.
Also, research flying into the vulnerability of VHF transmissions - ILS etc, demonstrated the difficulty in generating sufficiently confusing inputs so that both the technology and human would be deceived simultaneously.
The test conclusions were 'fortunatously' confirmed when the aircraft experienced VOR interference from an illegal radio transmission near Sheffield - hot wired / repeated from remote transmitters; the audio content could be heard on the intercom 'in tune' with the bearing deflection. Several instances / overflights were able identify the location, which the police successfully raided.