Add functional tests for flexible whitebind/list
Summary: This is a partial backport of Core PR16248 : https://github.com/bitcoin/bitcoin/pull/16248/commits/c5b404e8f1973afe071a07c63ba1038eefe13f0f
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5934