Add SLH-DSA FIPS self tests

Add SLH-DSA FIPS self tests

This requires a keygen test, as well as Sign/Verify tests for at least 1
sha2 algorithm and 1 shake related algorithm.

A pairwise consistency test has also been added to the key generation.

Note that self test datat for the signature is currently stored as a
sha256 digest in order to reduce the memory footprint.
(Since the signature size for sha2/shake using 128s = ~8K, and for 128f = ~17K)

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)