test: add functional test for anchors.dat
Summary:
It creates a node and adds 2 outbound block-relay-only connections and 5 inbound connections. When the node is down, anchors.dat should contain the 2 addresses from the outbound block-relay-only connections.
Backport of core#21338.
Ref T1696.
Test Plan:
./test/functional/test_runner.py feature_anchors
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1696
Differential Revision: https://reviews.bitcoinabc.org/D10959