core: frontend: vehicle-setup: allow testing servo and relay outputs

### Current behaviour

The PWM setup page has some parameters for configuring PWM and relay outputs, as well as motors, but only the motor outputs allow being tested. Testing servo and relay outputs currently requires a control station software or manual MAVLink messages, which is needlessly frustrating when you just want to check that an output is working as expected.

### Expected or desired behaviour

Add a servo and relay testing section, similar to the existing motor test section, but without the requirement to arm the vehicle to use it. Could bring #1818 into it, with PWM limit params specified around the test bar, like

### Prerequisites

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

Start at the PWM setup page and compare the existing motor test section with the requested servo and relay testing sections. Check how testing currently requires arming and how control-station software or manual MAVLink messages are used; done means servo and relay outputs can be tested from the page without arming the vehicle.

Written by the indexing model from the issue text.