Fix cuckoocache_tests -Wcast-align warnings

Summary:
At the time the test was added, there was no fast 256bit random
facility.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4667