tests: add check for decoding netlink get and new flags

tests: add check for decoding netlink get and new flags

* tests/netlink_route.c (test_nlmsg_flags): New function, checks
decoding of NETLINK_ROUTE get and new flags.
(main): Use it.
* tests/netlink_sock_diag.c (test_nlmsg_flags): New function, checks
decoding of NETLINK_SOCK_DIAG get flags.
(main): Use it.
* tests/netlink_xfrm.c: (test_nlmsg_flags): New function, checks
decoding of NETLINK_XFRM get and new flags.
(main): Use it.