crypto/cipher has well defined interfaces with plenty of tricky requirements (about aliasing, different lengths, state) that are hard to test for and easy to overlook.

cryptotest will be an interface test suite, like nettest. It caught a bug in x/crypto/internal/chacha20 and can replace a bunch of duplicated tests in the standard library, too.

It's useful to expose it in x/crypto for external implementations of the interface to use it. It will be vendored back.

Originally submitted as an internal package in https://golang.org/cl/102196