tests: don't depend on set element order

tests: don't depend on set element order

Pablo reported test failures because the order of returned set entries
is not deterministic.

This sorts set elements before comparision.
Patrick suggested to move ordering into libnftnl (since we could f.e.
also get duplicate entries due to how netlink dumps work), but thats a bit
more work.  Hence this quick workaround.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>