tools: testing: vsock: Test when server is bound but not listening

Whenever the server side of vsock is binding to the socket, but not
listening yet, we expect the behavior from the client to be identical to
what happens when the server is not even started.

This new test runs the server side so that it binds to the socket
without ever listening to it. The client side will try to connect and
should receive an ECONNRESET error.

This new test provides a way to validate the previously introduced patch
for making sure the server side will always answer with a RST packet in
case the client requested a new connection.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>