Enable openssl req -x509 to create certificates from CSRs

Enable openssl req -x509 to create certificates from CSRs

`openssl req -x509` has code allowing it to generate certificates from CSRs
as a replacement for `openssl x509`, but a bug prevents it from working
properly. -CA and -CAkey can now be passed to generate a CA-signed
certificate as documented in openssl-req(1).

Regression testing has been added to `openssl req`.

Fixes #17736.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17782)