In the RFC6979 test vectors, ensure private keys are correctly encoded

In the RFC6979 test vectors, ensure private keys are correctly encoded

The encoding of these keys is supposed to have a fixed length based on the curve (this enables constant time processing, see oss-security this week). Several of these have private scalars that have leading 0s, but were encoded with the 0s truncated off. This adds back the 0s.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28956)