Need pulseaudio based audio tests

[Kristin Chuang]

The current set of audio tests[1] in checkbox-snappy are for desktops. We need some form of generic audio test cases that depend on pulseaudio for Ubuntu Core.

Cases needed:
* Audio output tests for the following:
 (1) external monitors, e.g. HDMI and DP audio
 (2) external headset/speaker
 (3) internal speaker

* Audio input tests for the following:
 (1) external headset/mic
 (2) internal headset/mic

[1] https://git.launchpad.net/plainbox-provider-snappy/tree/units/audio/jobs.pxu

[Kristin Chuang (kristinchuang)]

Commands to be used in upcoming project for reference:
https://docs.google.com/document/d/1Xf3Y6SZiNc8aObOWkJlUSAslEG4YcGTbeAvCs_evJ-c/edit#heading=h.9spftpczvo72

[Gavin Lin (gavin.lin)]

Added manual tests for pulseaudio.

I have tried to make these tests automatic, but decided to fall back to manual ones because:
1. pulseaudio snap has not beed maintained for a long while, I have passed this information to project team and they will discuss if they want to stay with pulseaudio. So currently we don't want to spend more time to develop test case.
2. The sink/source id will vary on different devices so this will take more time to figure out how to output to correct device.

[Gavin Lin (gavin.lin)]

https://code.launchpad.net/~gavin.lin/plainbox-provider-snappy/+git/plainbox-provider-snappy/+merge/359807