test: Default initialize should_freeze to true

Summary:
It is safe now, when move semantics is used instead of a custom swap
function.

This is a partial backport of core#26749
https://github.com/bitcoin/bitcoin/pull/26749/commits/95ad70ab652ddde7de65f633c36c1378b26a313a

Depends on D16146

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16147