crypto,hash: add t.Helper calls to test helpers

Use t.Helper to make the reported failure lines more helpful.

Change-Id: I6593924b3892b2441a197aee4e05f71ea236d426
GitHub-Last-Rev: 2e020ea85cf44c6cc0e90cb919eb0d7bb5a1c315
GitHub-Pull-Request: golang/go#68843
Reviewed-on: https://go-review.googlesource.com/c/go/+/604755


Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>