Request: add duration to "Motor Test"

By turning on full logging and spinning each motor in turn, you can check your motors+props for vibration.

However, currently the time it spins each motor isn't long enough to get a good consistent reading. So on the "Motor Test" screen could we have a "Spin Duration" field alongside the "Throttle %" we currently have? Default that to the current spin time of course.

Start by locating the Motor Test screen and the code that currently controls how long each motor spins. Trace how the existing Throttle % value is presented and applied, then verify that a Spin Duration field uses the current spin time by default and changes the motor-test duration.

Written by the indexing model from the issue text.