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

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

This CL creates tests for the cipher.BlockMode interface in the new
cryptotest package.  This set of tests is called from the tests of
implementations of the BlockMode interface e.g. cbc_test.go

Updates #25309

Change-Id: I3685bbee24d08d66f5bb4b7f001cbf520c844881
Reviewed-on: https://go-review.googlesource.com/c/go/+/595120
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russell Webb <russell.webb@protonmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>