wireguard: selftests: reduce complexity and fix make races

This gives us fewer dependencies and shortens build time, fixes up some
hash checking race conditions, and also fixes missing directory creation
that caused issues on massively parallel builds.

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