Sounds like you are looking for a servo tester, or a servo programmer, sometimes called a servo driver. They can control an ESC like a RC radio system does.

A few examples, there are others but I can't seem to find them right now:

Servo Programmers


Note it takes more than just a potentiometer to control a brushless motor, as something needs to do the phase/commutation switching required. A brushless ESC or controller performs this function. Also a pot is generally significantly limited in the amount of power it can control.