test: fix constructing CNode with invalid inbound_onion
as CNode ctor should only be passed inbound_onion = true
when the connection is inbound
net: assert CNode::m_inbound_onion is inbound in ctor
and drop an unneeded check in CNode::ConnectedThroughNetwork()
net: add CNode::IsInboundOnion() public getter and unit tests
This is a backport of core#20210