ASoC: wm_adsp: Add kunit test for firmware file search

Add KUnit testing of the wm_adsp code that searches for firmware files.
Also make the SND_SOC_WM_ADSP Kconfig symbol visible if KUNIT is enabled
so that wm_adsp can be manually included for KUnit testing.

The firmware filename is composed of several fields, some of which are
optional, and there is a search algorithm to fallback from specific to
generic versions of firmware for a device. This KUnit test verifies that
wm_adsp is searching for the correct sequence of filenames.

The are two ways of testing this, and both are used in this KUnit test.

1. Trap the calls to firmware_request_nowarn() and test that the sequence
   of filenames request is correct.

   This is the most thorough test because it proves that exactly the
   expected filenames are requested, in the correct order.

   But it doesn't fully cover regression testing. If a change to the search
   algorithm changes the expected sequence of requested files, the test
   must also be changed to expect that new sequence. If the expectation is
   wrong, the tests can pass (because the search order is as expected)
   while picking a different file in some cases from what it did before the
   change.

2. Test which file is picked from a simulated directory of files.

   This is better for regression testing because it is independent of the
   search algorithm. It does not need to change if the search algorithm
   changes. It is not testing exactly which files the algorithm searches
   for, only which file it eventually picks from a given set of available
   files.

   In other words, the regression test is: does it still pick the same file
   from the same directory of files?

   But it is impractical for thorough testing. It doesn't prove that
   exactly the correct files were searched for, unless it was to test with
   every possible combination of file names and directory content that
   could ever exist. Clearly this is impossible to implement, since the
   number of combations of possible valid filenames in a directory and
   number of files in a directory is astronomically large.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20260310141817.1871794-5-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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