Using a Servo motor as load on a system
Geared servo motors generally do not like being run in reverse. You may damage the geartrain. Simple DC motors are better for that, although not at low speed. Connect a power resistor across the terminals and it will dump energy from the shaft speed into the resistor.

You might be better off with a mechanical friction brake of some kind. Or dump the energy into a fluid. A set of paddles rotating in the air is a common small test load. For lower speeds and higher torque, put a paddle in a liquid such as water or oil.