lib/test: convert lib/test_list_sort.c to use KUnit

lib/test: convert lib/test_list_sort.c to use KUnit

Functionally, this just means that the test output will be slightly
changed and it'll now depend on CONFIG_KUNIT=y/m.

It'll still run at boot time and can still be built as a loadable
module.

There was a pre-existing patch to convert this test that I found later,
here [1]. Compared to [1], this patch doesn't rename files and uses
KUnit features more heavily (i.e. does more than converting pr_err()
calls to KUNIT_FAIL()).

What this conversion gives us:
* a shorter test thanks to KUnit's macros
* a way to run this a bit more easily via kunit.py (and
CONFIG_KUNIT_ALL_TESTS=y) [2]
* a structured way of reporting pass/fail
* uses kunit-managed allocations to avoid the risk of memory leaks
* more descriptive error messages:
  * i.e. it prints out which fields are invalid, what the expected
  values are, etc.

What this conversion does not do:
* change the name of the file (and thus the name of the module)
* change the name of the config option

Leaving these as-is for now to minimize the impact to people wanting to
run this test. IMO, that concern trumps following KUnit's style guide
for both names, at least for now.

[1] https://lore.kernel.org/linux-kselftest/20201015014616.309000-1-vitor@massaru.org/
[2] Can be run via
$ ./tools/testing/kunit/kunit.py run --kunitconfig /dev/stdin <<EOF
CONFIG_KUNIT=y
CONFIG_TEST_LIST_SORT=y
EOF

[16:55:56] Configuring KUnit Kernel ...
[16:55:56] Building KUnit Kernel ...
[16:56:29] Starting KUnit Kernel ...
[16:56:32] ============================================================
[16:56:32] ======== [PASSED] list_sort ========
[16:56:32] [PASSED] list_sort_test
[16:56:32] ============================================================
[16:56:32] Testing complete. 1 tests run. 0 failed. 0 crashed.
[16:56:32] Elapsed time: 35.668s total, 0.001s configuring, 32.725s building, 0.000s running

Note: the build time is as after a `make mrproper`.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Tested-by: David Gow <davidgow@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>