selftests: router_bridge: Add remastering tests

Add two tests to deslave a port from and reenslave to a bridge. This should
retain the ability of the system to forward traffic, but on an offloading
driver that is sensitive to ordering of operations, it might not.

The first test does this configuration in a way that relies on
vlan_default_pvid to assign the PVID. The second test disables that
autoconfiguration and configures PVID by hand in a separate step.

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