RFE: inet_socket: test label of new connection sockets

[stephensmalley]

inet_socket: test label of new connection sockets, esp MLS attributes.
This is to test that new connection sockets are labeled correctly, i.e. we end up with the user:role:type from the listening socket and the MLS label of the SYN request (which should correspond to the peer's MLS label). To some extent, we might be getting this already via the peer context testing and the level-based connection tests, but need to at least confirm that it is adequately being tested.