Another young man discovers you need not be an expert machinist to succeed as a gear engineer. Spoiler alert: never tangle with a grinding wheel; they are undefeated in all time.
Historically, gearbox original equipment manufacturers (OEMs) and repair organizations have tended to offer their customers no-load, full speed (spin) tests as a standard performance test. If a load test was specified, the supplier would probably offer a locked torque back-to-back simulated load test, which requires a large investment in tooling to connect shafts of the test and slave gearboxes.