vehicle-setup: pwm-outputs: allow testing all configured motors

### Current behaviour

ArduSub support is [hardcoded to motors 1-8](https://github.com/bluerobotics/BlueOS/blob/master/core/frontend/src/components/vehiclesetup/PwmSetup.vue#L264), which prevents testing additional motors.

Desired by [at least this forum user](https://discuss.bluerobotics.com/t/rov-with-9-or-more-motors/23349), and presumably any others who are using BlueOS with a custom Sub vehicle that has more than 8 motors.

### Expected or desired behaviour

Allow testing all connected motors that are supported by ArduSub's parameters.

For now that's 1-12, though in future (with scripting-based motor support) we may want a more generic approach than hardcoded parameter values...

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start in core/frontend/src/components/vehiclesetup/PwmSetup.vue around line 264 and inspect how ArduSub motor testing is limited to motors 1-8. Extend the behavior to cover the currently supported motor range through 12, and verify that all configured supported motors can be tested.

Written by the indexing model from the issue text.