Update p2p_* functional tests to use the new .format() method
Summary: Avoid using % to format strings, switch to the new method instead.
Test Plan:
./test/functional/test_runner.py p2p_compactblocks p2p_leak
p2p_sendheaders
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2570