kselftest/alsa: Run PCM tests for multiple cards in parallel

With each test taking 4 seconds the runtime of pcm-test can add up. Since
generally each card in the system is physically independent and will be
unaffected by what's going on with other cards we can mitigate this by
testing each card in parallel. Make a list of cards as we enumerate the
system and then start a thread for each, then join the threads to ensure
they have all finished. The threads each run the same tests we currently
run for each PCM on the card before exiting.

The list of PCMs is kept global since it helps with global operations
like working out our planned number of tests and identifying missing PCMs
and it seemed neater to check for PCMs on the right card in the card
thread than make every PCM loop iterate over cards as well.

We don't run per-PCM tests in parallel since in embedded systems it can
be the case that resources are shared between the PCMs and operations on
one PCM on a card may constrain what can be done on another PCM on the same
card leading to potentially unstable results.

We use a mutex to ensure that the reporting of results is serialised and we
don't have issues with anything like the current test number, we could do
this in the kselftest framework but it seems like this might cause problems
for other tests that are doing lower level testing and building in
constrained environments such as nolibc so this seems more sensible.

Note that the ordering of the tests can't be guaranteed as things stand,
this does not seem like a major problem since the numbering of tests often
changes as test programs are changed so results parsers are expected to
rely on the test name rather than the test numbers. We also now prefix the
machine generated test name when printing the description of the test since
this is logged before streaming starts.

On my two card desktop system this reduces the overall runtime by a
third.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230203-alsa-pcm-test-card-thread-v1-1-59941640ebba@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

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