remove redundant str conversions in f-strings (test_framework/util.py)

Summary:
Some of these became unecessary after D13199 (previously we sometimes used string concatenation with the + operator)
Some of these were never necessary (the ones that previously used "%s" % (str(thing), ) or "{}".format(str(thing)))

Also replace one instance of f"{repr(thing)}" with the f"{thing!r}" shortcut.

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17319