tests: add test for closing TCP connection with pending packets

The test is supposed to trigger the problem from issue #259. I was not
able to trigger the problem using guest-local communication, hence the
client is external to the guest.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>