wireguard: selftests: ensure non-addition of peers with failed precomputation

Ensure that peers with low order points are ignored, both in the case
where we already have a device private key and in the case where we do
not. This adds points that naturally give a zero output.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>