This test helper can test any hash.Hash, not just crypto hashes. Move
it to $GOROOT/src/internal/testhash so both crypto and hash can use
it.

For #69521

Change-Id: Iac086cca513d5c03936e35d1ab55b8636f4652f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/670178


Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>