Add unit test for ChaCha20's new caching

Summary:
This concludes backport of core#26153
https://github.com/bitcoin/bitcoin/pull/26153/commits/511aa4f1c7508f15cab8d7e58007900ad6fd3d5d
Depends on D18826

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18827