selftests: router_bridge_lag: Add a new selftest

Add a selftest to verify that routing through a bridge works when LAG is
used instead of physical ports.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>