[tests] Move only: move NodeConnCB below NodeConn

Summary:
This is required since NodeConnCB will inherit from NodeConn
after the next commit.

Partial backport of core PR11712 (commit e30d404)

Depends on D2358

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2359