[tests] define NODE_NETWORK_LIMITED in test framework

Summary:
This makes the bitfields comparison easier to work with.

Partial Backport of core PR11867 (commit dbfe294)

Depends on D2366

Test Plan:

./test/functional/test_runner.py node_network_limited.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2367