[tests] Make NodeConnCB a subclass of NodeConn

Summary:
This makes NodeConnCB a subclass of NodeConn, and
removes the need for the client code to know
anything about the implementation details of NodeConnCB.

NodeConn can now be swapped out for any other implementation
of a low-level connection without changing client code.

Partial backport of core PR11712 (commit dad596f)

Depends on D2359

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2360