This commit adds testing and Known Answer Tests (KATs) to OpenSSL for
the `BN_gcd` function.
(cherry picked from commit b75d6310857bc44ef2851bde68a1979c18bb4807)
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10122)
