Add some simple KUnit tests for the nh() function.
These replace the test coverage which will be lost by removing the
nhpoly1305 crypto_shash.
Note that the NH code also continues to be tested indirectly as well,
via the tests for the "adiantum(xchacha12,aes)" crypto_skcipher.
Link: https://lore.kernel.org/r/20251211011846.8179-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
