selftests: forwarding: Add a test for VLAN-unaware bridge

Similar to the VLAN-aware bridge test, test the VLAN-unaware bridge and
make sure that ping, FDB learning and flooding work as expected.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>