Use DCOMConnection in test_RemoteGetClassObject instead of a raw
DCE/RPC connection from self.connect(). RemoteGetClassObject returns an
interface whose RemRelease path expects the DCOMConnection PORTMAPS cache
to be initialized for the target; the raw connection path leaves that
cache empty and can raise KeyError keyed by the target address.
Wrap the interface use in a try/finally so the DCOM connection is always
disconnected after the test.