net: Use mockable time for ping/pong, add tests
Summary:
Switch CNode::m_ping_start and CNetMessage::m_time to mockable time, so that tests can be added. Mockable time is also type-safe, since it uses std::chrono
Backport of core PR18638.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9129