Answer

Using the test scripts provided with BlueZ to setup a GATT server:

 cd /lib/bluez/test/ ./example-gatt-server & 

Power on Bluetooth, start advertising and set to pairable:

 # bluetoothctl Agent registered [bluetooth]# power on Changing power on succeeded [bluetooth]# advertise on [bluetooth]# pairable on 

Connect with Nordic's nRF smartphone app and read the fake battery level.