selftests: router_bridge_vlan: Add PVID change test

Add an alternative path involving VLAN 777 instead of the current 555. Then
add tests that verify that marking 777 as PVID makes the 555 path not work,
and the 777 path work.

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