Store the value of the P2P port used in each TestNode instances
Summary:
This ensure that we can get rid recomputing the port number based on state that comes from outside the TestNode instance.
Depends on D2247
Test Plan:
./test/functional/test_runner.py --extended
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2248