selftests: forwarding: mirror_gre_vlan_bridge_1q: Fix roaming test

ARP or ND traffic can cause spurious migration of FDB back to $swp3.
Mirroring is then updated in accordance with the change, and mirrored
packets are seen at h3, causing a failure.

Detect the case of this spurious roaming, and retry the test.

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