crypto/x509,embed: use slices to clean up tests

Replace reflect.DeepEqual with slices.Equal, which is much faster.

Change-Id: Ia93cc153d1f71ce92656129843be8dadcefbbca3
GitHub-Last-Rev: 0af0cc4205f51b31780ca2cbefe780e7256b1188
GitHub-Pull-Request: golang/go#67610
Reviewed-on: https://go-review.googlesource.com/c/go/+/587817


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>