Move the add_outbounds helper to the test object in p2p_add_connections.py

Summary:
This will make the next changes easier to review. There is no change in behavior.

Ref T1696.

Test Plan:

./test/functional/test_runner.py p2p_add_connections

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D11009