Add BN support to the test infrastructure.

This includes support for:

- comparisions between pairs of BIGNUMs
- comparisions between BIGNUMs and zero
- equality comparison between BIGNUMs and one
- equality comparisons between BIGNUMs and constants
- parity checks for BIGNUMs

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3405)