Add GCD testing infrastructure.

This commit adds testing and Known Answer Tests (KATs) to OpenSSL for
the `BN_gcd` function.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Re

Add GCD testing infrastructure.

This commit adds testing and Known Answer Tests (KATs) to OpenSSL for
the `BN_gcd` function.

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)

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