Various simplification and improvement to ABC integration test

Summary:
As per title.

Depends on D1143

Test Plan:

./test/functional/test_runner.py abc-p2p-compactblocks abc-p2p-fullblocktest abc-mempool-accept-txn

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1144