Add SLH_DSA key validation.

Add SLH_DSA key validation.

The pairwise test requires that the computed PK_ROOT key matches the
keys PK_ROOT value. The public and private key tests just require the
key elements to exist.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25882)