test: added simpositioner with failure signal

Description

Added test device to simulate a device failure. The failure can be set through the device.fails signal:

• 0: no failure
• 1: comm failure with raised error
• 2: comm failure but no response

Example:

dev.failure_device.fails.set(0).wait()

A typical config would be:

failure_device:
  deviceClass: sim:sim:SimPositionerWithCommFailure
  deviceConfig:
    delay: 1
    update_frequency: 400
  enabled: true
  readOnly: false
  readoutPriority: baseline

Definition of Done

• Documentation is up-to-date.