net: remove erroneous Dial check in TestListenerClose
TestListenerClose had been asserting that a Dial to the newly-closed
address always fails, on the assumption that the listener's address
and port would not be reused by some other listener that could then
accept the connection.
As far as I can tell, that assumption is not valid: the Dial after
Close may well connect to a Listener opened for some other test, or
even one opened by a completely different process running concurrently
on the same machine.