check all changes to SLH-DSA after 3rd round are incorporated

[falko-strenzke]

for the list of changes, see: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.205.ipd.pdf#page=12

[TJ-91]

Indeed, I missed / wrongly dismissed PR 51 of https://github.com/sphincs/sphincsplus/

I fixed this and updated the test vectors in the all-pqc branch.

The issue is brought up on the mailing list here https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/88tuvtb7nN4/m/DA1QCoJWBAAJ andit doesn't seem to be fully resolved, at least the PR is not merged to main yet and there might be further changes in the future.