This completes backport of core#22697.
CAddrMan::Clear() exists to reset the internal state of CAddrMan. It's currently used in two places:
on startup, if deserializing peers.dat fails, Clear() is called to reset to an empty addrman
in tests, Clear() is called to reset the addrman for more tests
In both cases, we can simply destruct the CAddrMan and construct a new, empty addrman. That approach is safer - it's possible that Clear() could 'reset' the addrman to a state that's not equivalent to a freshly constructed addrman (one actual example of this is that Clear() does not clear the m_tried_collisions set). On the other hand, if we destruct and then construct a fresh addrman, we're guaranteed that the new object is empty.
This wasn't possible when addrman was initially implemented, since it was a global, and so it would only be destructed on shutdown. However, addrman is now owned by node.context, so we have control over its destruction/construction.This involves reverting D11827 in order to match core and be a bit creative with the tests.