`Misbehaving` has several coding related issues (ignoring the conceptual
issues here for now):
* It is public, but it is not supposed to be called from outside of
net_processing. Fix that by making it private and creating a public
`UnitTestMisbehaving` method for unit testing only.
* It doesn't do anything if a `nullptr` is passed. It would be less
confusing to just skip the call instead. Fix that by passing `Peer&`
to `Misbehaving()`.
* It calls `GetPeerRef`, causing `!m_peer_mutex` lock annotations to be
propagated. This is harmless, but verbose. Fix it by removing the no
longer needed call to `GetPeerRef` and the no longer needed lock
annotations.
This is a backport of core#25144
See D384 for why we already had an overloaded Mishbehaving for `CNode`