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

This CL creates tests for the cipher.AEAD interface in the new
cryptotest package.  This set of tests is called from the tests of
implementations of the AEAD interface, such as the GCM blockmode.

Updates #25309

Change-Id: I7612fa6fb6c1505bdf1a2cd71180dd43dc50bf4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/601778


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russell Webb <russell.webb@protonmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>