Update abc-* 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 abc-cmdline

abc-magnetic-anomaly-mining abc-p2p-compactblocks abc-p2p-fullblocktest
abc-parkedchain abc-replay-protection abc-rpc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2567