selftests: mlxsw: Use permanent neighbours instead of reachable ones

The nexthop objects tests configure dummy reachable neighbours so that
the nexthops will have a MAC address and be programmed to the device.

Since these are dummy reachable neighbours, they can be transitioned by
the kernel to a failed state if they are around for too long. This can
happen, for example, if the "TIMEOUT" variable is configured with a too
high value.

Make the tests more robust by configuring the neighbours as permanent,
so that the tests do not depend on the configured timeout value.

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