wireguard: selftests: tie socket waiting to target pid

Without this, we wind up proceeding too early sometimes when the
previous process has just used the same listening port. So, we tie the
listening socket query to the specific pid we're interested in.

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