Improve test vectors for ChaCha20
Summary:
This is a partial backport of core#26153
https://github.com/bitcoin/bitcoin/pull/26153/commits/fb243d25f754da8f01793b41e2d225b917f3e5d7
Depends on D18825
Test Plan: ninja check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18826