reset m_best_fork_tip if the fork becomes the main chain
This fixes a spurious "CheckForkWarningConditions: Warning: Large fork found" log when a block is unparked resulting in m_best_fork_tip now being added back to the main chain. This warning kept being logged on every new block even though the fork was resolved.
It also adds a functional test for that log message which was previously untested.
Test Plan: ninja check check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13909