test: Avoid connecting to real network when running tests

Summary:
Backport of core#21254.

This is the complete fix for the p2p_dos_header_tree.py test, so the temporary fix introduced in D10908 can be reverted.

Depends on D10909.

Test Plan:

ninja all check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10911