Fix abc-p2p-fullblocktest.py

Summary: iImports are fubared.

Test Plan:

../qa/pull-tester/rpc-tests.py abc-p2p-fullblocktest.py

Reviewers: freetrader, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D215