test: use f-strings in rpc_* tests

Summary:
scripted diff

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

Manual edits:

• remove unnecessary string formatting (f"{-1}" -> "-1")
• convert more "...".format(..) instances to f-strings. These were missed by flynt presumably because it sometimes cannot understand multiline strings formatted by autopep8

Depends on D13196

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13197