[tests] Add network_thread_ functions and use them in tests

Summary:
Add network thread_start(), network_thread_running() and
network_thread_join() utility functions in mininode.py

Update the tests to use them.

Partial backport of core PR11849
commits 5fc6e71 and 74e64f2

Depends on D2370

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2383