test: use f-strings in wallet_* tests

Summary:
scripted diff

flynt --transform-concats test/functional/wallet_*

Depends on D13197

Test Plan: test/functional/test_runner.py wallet_*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13198