test: use f-strings in interface_* tests

Summary:
scripted diff

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

Depends on D13193

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13194