kselftest/alsa: pcm - Add more coverage by default

Add more coverage to our standard test cases:

 - 8kHz mono and stereo to verify that the most common mono format is
   clocked correctly.
 - 44.1kHz stereo to verify that this different clock base is generated
   accurately.
 - 48kHz 6 channel to verify that 6 channel is clocked correctly.
 - 96kHz stereo since that is a common audiophile rate.

Signed-off-by: Mark Brown <broonie@kernel.org>