selftests: add rtnetlink test script

add a simple script to exercise some rtnetlink call paths, so KASAN,
lockdep etc. can yell at developer before patches are sent upstream.

This can be extended to also cover bond, team, vrf and the like.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>