Add RFC7748 6.1. Curve25519 Test vector

[shampoofactory]

Hi all. Whilst RFC7748 5.2. Test vector tests are present, RFC7748 6.1. Curve25519 Test vector tests appear to be missing. I've added them in.

I was unsure as to which branch to open a pull request to, so I've just opened it against main.

Thank you.

Reference: https://www.rfc-editor.org/rfc/rfc7748#section-6.1