I bought a gizmo called TXServo a couple of years back. It connects to a PC via USB and comes with a program that allows you to run various "scenarios". I never referred to it as burning in a servo, but I have used it to center the servos.

The device has four slots that look pretty much like an RX servo plug-in area and I can "exercise" four servos at once.

As I said, I've used it to center servos prior to doing various connections and adjustments to the linkage.

The below is a screen capture from their web page.

CGr.