First, those drivers are not... robust... you can fry them. One of the quickest ways to fry any driver is to disconnect a motor wire while the unit is powered on. The current built up in the coil collapses and causes a massive voltage spike through the other leads on the motor and takes out the driver. So I would really recommend against using a breadboard since the connections are less than totally reliable. Instead, connect the motor and other wires /directly/ to the driver pins. This assumes you have female housings on the end of the wires (it looks like you do) but if not, I would strongly recommend investing in a crimper, pins and individual housings. And then tape around the individual housings so they become mechanically all one unit.
You did a valid test with the motor, great job troubleshooting! I would do the same sort of test with the output of the Arduino sketch after changing it so it only produces a step about once a second. Make sure that output is going up and down. And then I would make sure the driver had everything it needs. Are you pulling the Enable pin to the correct level? Go around the pin description on the datasheet and make sure each pin is where it needs to be. Once you know you are feeding it everything it should want, put a meter on each pin from the driver to the motor and feed in pulses slowly, like 2 a second. The meter should show a ramping up, then back down. Do that for each of the 4 motor wire pins. If you are sure you are feeding it everything it needs, and it isn't putting out a correct signal, then replace it. Your taped together pins will come off in one part, and stay together so they go on the right way on the new driver. Be sure to apply logic power FIRST, and THEN motor power. Read the datasheet to see why.
Hope that helps, and contact my site when you are ready for a real stepper driver!