This is a case for a $10 (or less) PWM you can get on Amazon and put together yourself with those maker skills: PWM: https://smile.amazon.com/gp/product/B00RYRXFW2/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1 Servo Screws: https://www.trimcraftaviationrc.com/index.php?route=product/product&path=63&product_id=388

With this, some wire, and some terminations, you can make yourself a nice manual motor controller for operating and testing motors. The PWM has power control and directional switch. This is about the cheapest way to do this short of connecting your motors straight to a battery.

Feel free to customize however you want. Native files attached.