selftests: forwarding: lib.sh: Validate NETIFS

The variable should contain at least NUM_NETIFS interfaces, stored
as keys named "p$i", for i in `seq $NUM_NETIFS`.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>