test: use f-strings in abc-* tests

Summary:
scripted diff

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

Depends on D13191

Test Plan: test/functional/test_runner.py abc-*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13192