curve25519: add test vectors from BoringSSL

This diff extends the curve25519 test suite with some test vectors
generated from BoringSSL.

Updates golang/go#30095

Change-Id: I4e871378c77b46167d162e9eec75fc412596ff7c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/161277


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>