lib: util_macros_kunit: add kunit test for util_macros.h

A bug was found in the find_closest() (find_closest_descending() is also
affected after some testing), where for certain values with small
progressions of 1, 2 & 3, the rounding (done by averaging 2 values) causes
an incorrect index to be returned.

The bug is described in more detail in the commit which fixes the bug. 
This commit adds a kunit test to validate that the fix works correctly.

This kunit test adds some of the arrays (from the driver-sphere) that seem
to produce issues with the 'find_closest()' macro.  Specifically the one
from ad7606 driver (with which the bug was found) and from the ina2xx
drivers, which shows the quirk with 'find_closest()' with elements in a
array that have an interval of 3.

For the find_closest_descending() tests, the same arrays are used as for
the find_closest(), but in reverse; the idea is that
'find_closest_descending()' should return the sames indices as
'find_closest()' but in reverse.

For testing both macros, there are 4 special arrays created, one for
testing find_closest{_descending}() for arrays of progressions 1, 2, 3 and
4.  The idea is to show that (for progressions of 1, 2 & 3) the fix works
as expected.  When removing the fix, the issues should start to show up.

Then an extra array of negative and positive values is added.  There are
currently no such arrays within drivers, but one could expect that these
macros behave correctly even for such arrays.

To run this kunit:
  ./tools/testing/kunit/kunit.py run "*util_macros*"

Link: https://lkml.kernel.org/r/20241105145406.554365-2-aardelean@baylibre.com


Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Cc: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>