lib/prime_numbers: convert self-test to KUnit

Extract a private header and convert the prime_numbers self-test to a
KUnit test. I considered parameterizing the test using
`KUNIT_CASE_PARAM` but didn't see how it was possible since the test
logic is entangled with the test parameter generation logic.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250208-prime_numbers-kunit-convert-v5-2-b0cb82ae7c7d@gmail.com


Signed-off-by: Kees Cook <kees@kernel.org>