UPSTREAM: 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>
(cherry picked from commit 04ddf1208f03e1dbc39a4619c40eba640051b950)
Bug: 152722841
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I842d8f3e2fbe3cf29d50d2bc9463299e74c7aef1