selftests: forwarding: Add a test case for externally learned FDB entries

Test that externally learned FDB entries can roam, but not age 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>