crypto/internal/cryptotest: fix typos in AEAD test comments

crypto/internal/cryptotest: fix typos in AEAD test comments

Some comments that describe the behavior being tested don't match the
test logic.

Fix the comments to match test logic and documented behavior.

Change-Id: I065d9d9069fa00bb865209f48e112456384c2b9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/616338
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>