selftests: router_bridge: Add tests to remove and add PVID

This test relies on PVID being configured on the bridge itself. Thus when
it is deconfigured, the system should lose the ability to forward traffic.
Later when it is added again, the ability to forward traffic should be
regained. Add tests to exercise these configuration changes and verify
results.

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