test: use f-strings in feature_* tests (part 2).
Summary:
This is a partial backport of core#22229
https://github.com/bitcoin/bitcoin/pull/22229/commits/a2502cc63fd308be8af840962da9c53339433fa6
https://github.com/bitcoin/bitcoin/pull/22229/commits/a2de33cbdc79202bccddb4beadfde88266ac979f
https://github.com/bitcoin/bitcoin/pull/22229/commits/ff7e3309995a8960ac371741b2b00c6da40f7490
https://github.com/bitcoin/bitcoin/pull/22229/commits/e9ca8b254d4b9567831c0e113ce1c0a2b4795a95
https://github.com/bitcoin/bitcoin/pull/22229/commits/fb633933ab570e945d2a366f37eeff39f516c613
Test Plan: ninja check-functional-extended
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13187