crypto/ed25519: add comprehensive edge-case test vectors

This will allow us to make changes to the internals confidently, without
risking causing issues in consensus applications. It will also prevent
architecture-specific divergence, like #40475.

Fixes #40478

Change-Id: I8c2b31406ca88add6941f14d8df8cecb96379cde
Reviewed-on: https://go-review.googlesource.com/c/go/+/304349


Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Katie Hockman <katie@golang.org>