selftests: rtnetlink: Test bridge enslavement with different parent IDs

Test that an upper device of netdevs with different parent IDs can be
enslaved to a bridge.

The test fails without previous commit.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>