I'm with this guy! A servo controller is all you would need, considering you use the same size/brand of motor/ESC.
As far as not interrupting power to ESC while not wanting motors revved up, I'd say find a good speed with the servo controller then place the rev switch on the red wire coming off the ESC (The power source for the servo tester) And if that won't work maybe place the rev switch on the black or white wires so the servo tester stays on, but you interrupt it's communication to the ESC...
Thanks. That's what I think I'll do.