I recommend you do some research. For instance,
Basically a testbed is a testing system that can exercise (stimulate) another system with certain inputs (actions, etc) and compare that system's outputs with expected outputs. A simulator is a system that simulates the behavior of another system. For instance, a flight simulator is a device that simulates the flight of an aircraft.
As a result, you could say that a testbed is a system that "simulates" user input but this would not be accurate: it may stimulate (note stimulate not simulate) a system with inputs, but it doesn't pretend to be the user or attempt to predict what the user will do next, rather it simply takes the outputs and compares them to expected outputs.