selftests: router_bridge_1d: Add a new selftest

Add a selftest to verify that routing through a 1d bridge works when VLAN
upper of a physical port is used instead of a physical port. Also verify
that when a port is attached to an already-configured bridge, the
configuration is applied.

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