This makes the test more robust by:
1. Decreasing the '1' threshold during calibration - the RMS value for
the sine wave will be 0.5, so the previous code was making us take
the ALSA mixer past 0dB.
2. Using the difference rather than absolute value for 0->1 transitions,
so that we're somewhat independent noise in our calculations.
This test is intended to measure real latency by playing a sample to a
sink and capturing that over a loopback interface. The loopback can
either be physical (cable running from headphone out to line in) or
virtual (monitor source or module loopback).
Also included in this is calibration code to make sure that volumes are
sufficiently adjusted to be able to detect the played back signal (and
that there aren't false positives due to line noise).
One of the objectives of all this is to later factor out the setup code
to allow us to easily write more loopback tests for various
functionality (volumes, resampling, mixing, etc.).
