zdtm: check sockets in the TCP_CLOSING state

TCP_CLOSING both sides have shutdown but we still have
            data we have to finish sending

travis-ci: success for series starting with [01/21] build: install libnet-dev
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>