selftests: mlxsw: Test unresolved neigh trap with resilient nexthop groups

The number of nexthop buckets in a resilient nexthop group never
changes, so when the gateway address of a nexthop cannot be resolved,
the nexthop buckets are programmed to trap packets to the CPU in order
to trigger resolution. For example:

 # ip nexthop add id 1 via 198.51.100.1 dev swp3
 # ip nexthop add id 10 group 1 type resilient buckets 32
 # ip nexthop bucket get id 10 index 0
 id 10 index 0 idle_time 1.44 nhid 1 trap

Where 198.51.100.1 is a made-up IP.

Test that in this case packets are indeed trapped to the CPU via the
unresolved neigh trap.

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