selftests: router_bridge_vlan_upper_pvid: Add a new selftest

This tests whether changes to PVID that coincide with an existing VLAN
upper throw off forwarding. This selftests is specifically geared towards
offloading drivers, but since there's nothing HW-specific in the test
itself (it absolutely is supposed to pass on SW datapath), it is put into
the generic forwarding directory.

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