There's now almost a sub-industry of testing & the units required for this. The main item is known in our company as a "System Integration Test System (or Rig) SITS or SITR. This usually contains both the main Avionics (AP, FMS, sensors, displays, Input/Output devices etc) plus a radio rack and sub-system rigs such as Flight Control units, Stall Computers, FADECs and so on. Backing all these, are simulation computers to allow the system to "fly" and to set individual inputs & discretes for logic verification etc.
Some companies have plans for complete rigs so that the entire integrated systems can be tested before the airframe is even built.