Ignore additional return codes in the netmgr unit tests

Ignore additional return codes in the netmgr unit tests

There was inconsistency in which error codes would get accepted and
ignored in the network manager unit test callbacks.  Add following
results, so we just detach the handle instead of causing assertion
failure:

* ISC_R_SHUTTINGDOWN - when the network manager is shutting down
* ISC_R_CANCELED - the socket has been shut down
* ISC_R_EOF - the (TCP) communication has ended on the other side
* ISC_R_CONNECTIONRESET - the TCP connection was reset

This should fix some of the spurious unit test failures.