[backport#16465 3/3] test: minor refactoring

Summary:
The initial commit added tests that are not relevant for us,
but also did some minor refactoring that could add more robustness
if the tests evolve in the future (remember extra_args in
setup_network in nodes are restarted).

The initial commit introduced a bug later fixed in PR16920.
This fix taken into accound here.

This concludes backport of Core PR16465 - part 3 of 3
Commit https://github.com/bitcoin/bitcoin/pull/16465/commits/fa25668e1c8982548f1c6f94780709c625811469
Depends on D8055

Test Plan: ninja && test/functional/test_runner.py wallet_import_rescan.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8056