test: use f-strings in p2p_* tests

Summary:
scripted diff

flynt --transform-concats test/functional/p2p_*

Manual changes:

• remove the + operator in a string concatenation so that flynt --transform-concats does not touch it (and therefore does not remove useful comments)
• remove unecessary string formatting when converting blockhashes to int

Depends on D13195

Test Plan: test/functional/test_runner.py p2p_*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13196