testing/infra/emulator: remove qemu warnings about audio

The default audio backend for qemu is configured at compile time. It
generates annoying warning messages to qemu's stderr when running our
tests, like these:
pulseaudio: set_sink_input_volume() failed
pulseaudio: Reason: Invalid argument
pulseaudio: set_sink_input_mute() failed
pulseaudio: Reason: Invalid argument

Explicitly set the audio backend to "none" at runtime to remove those
messages from our logs. There is no command line argument for this, so
use an environment variable when starting qemu.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>