lib/random32: convert selftest to KUnit

Convert the existing prandom selftest (lib/random32.c) to use the KUnit
framework (lib/tests/random32_kunit.c).  Unlike typical KUnit tests, this
file is directly #included into lib/random32.c.

The new test:
- Removes the legacy CONFIG_RANDOM32_SELFTEST from lib/random32.c.
- Adds CONFIG_PRANDOM_KUNIT_TEST (defaulting to KUNIT_ALL_TESTS).
- Moves the test logic to lib/tests/random32_kunit.c.

This commit is verified by `./tools/testing/kunit/kunit.py run`
with the .kunit/.kunitconfig:

CONFIG_KUNIT=y
CONFIG_PRANDOM_KUNIT_TEST=y

Link: https://lore.kernel.org/20260703050100.23944-1-note351@hotmail.com
Signed-off-by: Kir Chou <note351@hotmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jakub Kacinski <kuba@kernel.org>
Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>