There are many servo testers and even some servo centering tools available on the market. I didn’t find though the one I wanted to have, with selectable 1500/1520µs PWM outputs and also with 760/960µs outputs for helicopter fast tail servos.
The MAK RC Heli Servo Centering Tool may be especially interesting for RC helicopter modellers, due to its fast tail servo PWM output. Of course Flybarless controller unit can be always used to center servos, but I like to mount FBL unit at very end, without bothering about it during the build process.