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