selftests: forwarding: vxlan_bridge_1d: Add learning test

Add a test which checks that the VxLAN driver can learn FDB entries and
that these entries are correctly deleted and aged-out.

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