zdtm: skip 'tcp-close' tests if restore is skipped

'tcp-close' is a option that replaces connected tcp-sockets with
'closed' ones during restoring, therefore it have no sense without
restore stage. Thus let's skip related tests if the 'norst' flag was
specified.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>