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

[ Upstream commit 479a2b761d61c04e2ae97325aa391a8a8c99c23e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>