crypto: kdf - skip self-test when tests disabled

Make kdf_sp800108 honor the CONFIG_CRYPTO_MANAGER_DISABLE_TESTS kconfig
option, so that it doesn't always waste time running its self-test.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>