Add parameter to supply fixed ephemeral key for testing

[Frando]

snow, a Rust lang implementation of NOISE, has a method to supply a fixed ephemeral keypair, for test-only purposes. This patch adds a state key fixedE that can only be manually set after initialitation and is intended for test-only usage. It allows to achieve the exact same results when all parameters are the same.

[Frando]

Just leaving this here for the moment for reference:
I built the same test as added here in Rust with snow and, as expected after my tests in rust-node-noise-handshake: Messages 1 (<- e) and 2 (-> e, ee, s, es) are the same, but the last message (-> s, e) differs:

c29fb236b6ec95b4d3d1d1020bb0bf8beb24577bf291d9ec1469a9a79ad04dd12bb6ca5cccd6f65ec3933222fe51d68bb395d58fb8fadfb1399bd998af642382

To run the Rust test and see this result:

git clone git@github.com:Frando/snow.git
git checkout -t origin/xchachapoly-xxtest
cargo test xx_xchachapoly_blake2b -- --nocapture

[emilbayes]

Nice, I was actually going to implement this as it's a part of the fallback handshake, but in a slightly different way :) Perhaps I will base it off this code