[tests] bind functional test nodes to 127.0.0.1

Summary:
Prevents OSX firewall
allow-this-application-to-accept-inbound-connections
permission popups and is generally safer.

To prevent binding to 127.0.0.1, set self.bind_to_localhost_only =
False.

Backport of core PR12482
https://github.com/bitcoin/bitcoin/pull/12482/files

Depends on D3090 (prevent failure in abc-high_priority_transaction, see the CI build log).

Test Plan:

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

Also run on OSX

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3084