test: use f-strings in feature_* tests (part 3, with flynt)

Summary:
Inspired by core#22229 (see D13187 and D13184)

Scripted diff:

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

Manual changes:

• replace a "{1}" with "1"
• remove explicit string conversion inside placeholders (f"{str(foo}" -> f"{foo}")

Depends on D13190

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13191