[tests] fix flake8 warnings in node_network_limited.py

Summary:
This cleans up the comments and make the variables/functions conform to
python snake_case.

Partial backport of core PR11867 (commit 1285312)

Depends on D2363

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/D2366