selftests: forwarding: Test routed bridge interface

Add test for cases where bridge itself acts as a router interface, with
front panel port attached to the bridge in question.

In the first test (router_bridge.sh), VLAN memberships are not
configured in any way, and everything uses default PVID of 1. Thus
traffic in $h1 and $h2 is untagged. This test ensures that the previous
patches didn't break a currently working scenario.

In the second test (router_bridge_vlan.sh), a VLAN 555 pvid untagged is
added to the bridge CPU port, with that VLAN leaving the bridge tagged
through its sole member port. The traffic is therefore expected to come
out tagged at $h1. This tests the fix introduced in the previous
patches.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>