test: remove unnecessary block rehashing prior to solving

Summary:
Solving a block involves continously rehashing it, i.e. any extra
calls to rehash it before are not necessary and can be dropped.

This is a backport of core#23498

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17690