Add E2E tests with OpenSSL

[Sean-Der]

Add some basic testing using s_client + s_server

The author should also refactor/re-use our existing E2E testing code so that we don't duplicate in both places.

[Sean-Der]

Also introduce packet loss on either side. Create a Client/Server from a wrapped Conn and add a certain % of packet loss (it may be helpful to log messages, otherwise fixing will be really hard here)

[backkem]

We should probably implement these types of tests globally so we can throw them at any layer of the stack. The beginnings of this are here. Eventually it could look like this.

[igolaizola]

OpenSSL tests have been added in #193

TODO:

• Make OpenSSL work with ed25519 test

[at-wat]

I found that our implementation of Ed25519 signature calculation and hash used in ServerKeyExchange/CertificateVerify messages are wrong. Since both of our server and client have same bug, they can communicate each other.
I have made a fix for it, and now I'm cleaning the code.

One bad news is that OpenSSL haven't support DTLS Ed25519 yet.
I could confirm my fix works with this openssl branch: openssl/openssl#11115

[at-wat]

Ed25519 implementation of the master branch should be fixed now. We can enable the E2E test once OpenSSL supports DTLS 1.2 Ed25519.