use f-strings in functional tests (long lines)

Summary:
These lines were ignored by flynt in previous scripted-diffs because of the multiline nature.

I manually edited these lines to put them on a single line, ran flynt again, then manually formatted the resulting string.

There are more of these lines in test_runner.py, in test_framework/*.py files, in src/test/fuzz/test_runner.py and in test/util/bitcoin-util-test.py. These will be addressed in a follow-up to avoid making this diff even larger.

Depends on D13202

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13209