Add test for EC_KEY_set_private_key()

This tests the behavior and API of the EC_KEY_set_private_key function.
It tests compliance with legacy features related to NULL private keys
too.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/18941)