selftests: rtnetlink: add small test case with 'promote_secondaries' enabled

This exercises the 'promote_secondaries' code path.

Without previous fix, this triggers infinite loop/soft lockup:
ifconfig process spinning at 100%, never to return.

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