tests: Add socket_listen6 and socket6_udp as passing

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
log: Add pr_warn_once helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
sockets: Add warning about netlink sockets

They are not fully supported in OpenVZ.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
tests: Add notes about unability to start some tests

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>