Don't use default period-size and buffer-size #329

We need to control better what the period and buffer sizes are in all sof-tests.

The default values I see in the SoundWire tests make no sense

  buffer_size  : 16368
  period_size  : 4080
  period_time  : 85000

85ms for a period-time is not representative of any reasonable usage.

aplay and arecord can be configured with --period-time=5000 and --buffer-time=10000 for e.g 2 periods of 5ms.

It would be good enough to have an environment variable for each value, so that we can configure the tests at will, and even set stricter values for specific tests. I believe 5ms and 2 periods are decent baselines.