test: resort imports

Summary:
This is a partial backport of core#19760 [3/4]
https://github.com/bitcoin/bitcoin/pull/19760/commits/5e8df3312e47a73e747ee892face55ed9ababeea

Some changes are debatable (e.g. assert_is_hex_string > assert_is_hash_string), but I think it is best to deviate as little as possible from the Core code for such minor points of detail.

Depends on D9573

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9574