[backport#16551] test: Test that low difficulty chain fork is rejected
Summary:
https://github.com/bitcoin/bitcoin/pull/16551/commits/333317ce6b67aa92f7363d48cd750712190b4b6b
Concludes backport of Core PR16551
Test Plan:
./test/functional/test_runner.py p2p_dos_header_tree.py
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6431