test: Misc refactor to get rid of &foo[0] raw pointers
Summary:
Simplify test code
Backport of core#21840.
Depends on D18312.
Test Plan:
ninja check ninja bench-bitcoin
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D18313