You can not instantly reverse direction on a stepper motor. You have to decelerate, switch direction and accelerate again. The inputs to a stepper driver IC are just digital inputs, and there are a lot of options.
1. Put a uC on a breadboard and put GRBL in it (many versions for different uC's available). (Or write some simple test routines yourself).
2. Hook it up to a function generator. (Or NE555 or similar)
3. Combine it with a rotary encoder. Note that the regular 2-bit grey encoders can be connected directly to a stepper motor's step and direction pulses. You will loose some steps due to jitter, glitches and the exact position in which you switch direction, but it may be enough for some tests.