[PATCH v2] kselftest: alsa: Add simplistic test for ALSA mixer controls kselftest

[Pierre-Louis Bossart]

>>> +// This test will iterate over all cards detected in the system, exercising
> 
>> would it make sense to test only specific cards? People doing automated
>> tests might have a USB device for capture of analog loopbacks, or
>> injection of specific streams for capture, and usually care about
>> testing such devices - which do need manual setups and wiring btw.
> 
> It's not really idiomatic for kselftest to require any per system
> configuration by default - half the thing is that you can just run it
> and it should do as much as it can sensibly on the system.  You could
> definitely add some command line options for development or manual usage
> though.

I was thinking of adding this test to our CI, it's a bit orthogonal to
self-tests indeed. IIRC we check that we can modify all the PGA settings
for volume control but this test is a lot more generic.