tests: usb: uac2: Test Feature Unit descriptor

Insert Feature Units in between:
  * USB Streaming Input Terminal and Headphones Output Terminal
  * Microphone Input Terminal and USB Streaming Output Terminal

The Feature Units have Mute control only on the Primary channel 0.
The headphones Feature Unit also has Automatic Gain control on all
channels while microphone Feature Unit has Automatic Gain control only
on the Logichal channel 1.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>