Fix cache backend encryption unit test regressions.
Review Request #12408 — Created June 24, 2022 and submitted
A last-minute change was made to the new cache backend encryption
functionality to require byte strings for AES keys instead of allowing
for Unicode keys. While this decision might be reversed, the tests were
for sure wrong. They were hard-coding a key instead of using the
pre-defined constant for that key.
This change fixes the test regressions by using the correct constant.
Unit tests pass.