Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().

Add a new and slow unit test that checks if libscrypt_scrypt() and
EBP_PBE_scrypt() yield the same keys from test vectors.

squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().

squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().

squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().