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 <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
Run-TryBot: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go