crypto/x509: reduce boring test key size

crypto/x509: reduce boring test key size

Generating 8192 bit keys times out on builders relatively frequently. We
just need something that isn't a boringAllowCert allowed key size so we
can test that a non-boringAllowCert signed intermediate works, so just
use 512 instead since it'll be significantly faster.

Fixes #56798

Change-Id: I416e0d8c3aa11ff44e9870755efa95c74d1013f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/451656
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>