Fix intermittent issue in wallet_import_rescan & wallet_send

Summary:

test: Fix intermittent issue in wallet_import_rescan

test: Fix intermittent issue in wallet_send

This is a backport of core#20179

Test Plan: test/functional/test_runner.py wallet_send wallet_import_rescan

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10540