selftests: router_bridge_1d_lag: Add a new selftest

Add a selftest to verify that routing through several bridges works when
LAG VLANs are used instead of physical ports, and that routing through LAG
VLANs themselves works as physical ports are de/enslaved.

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