Precision angle (0.5° or better) with a motor
If your objective is to test the accuracy of the encoder (as a system together with the magnet), what you're proposing with the stepper or servo sounds like a bit of a red herring.  Since you are already apparently not trusting datasheets, why should you trust the specified accuracy of the servo or stepper?

I would suggest a simpler approach: attach a laser pointer to the same shaft as the magnet, letting the laser shine on a surface a couple of meters away.  Gradually turn the shaft and measure the distance that the dot moves for a given number of steps in the encoder output.  Some simple trig will give you the actual angle.  You can repeat that for a few different angular positions, and across the expected operating temperature range.