selftests: forwarding: mirror_gre_changes: Fix waiting for neighbor

When running the test on soft devices, there's no mechanism to
gratuitously start resolving the neighbor for remote tunnel endpoint.
So instead of passively waiting, wait for the device to be up, and then
probe the neighbor with a ping.

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