Set payload format back to JSON when done with func tests.
Set payload format back to JSON when done with func tests.

This clears the way for the BT tests to run after USB, since protobuf
commands aren't working over BT yet.

openxc/vi-firmware,ene-ilies/vi-firmware,mgiannikouris/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,mgiannikouris/vi-firmware,openxc/vi-firmware