crypto/internal/cryptotest: add tests for the cipher.Block interface

crypto/internal/cryptotest: add tests for the cipher.Block interface

This CL creates tests for the cipher.Block interface in the cryptotest
package.  This set of tests is called from the tests of implementation
of the Block interface e.g. aes_test.go and des_test.go.

Updates #25309

Change-Id: Ieea3752147c8163fc73a849cfcb8fa011205d2c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/594018
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russell Webb <russell.webb@protonmail.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>