Expand command-response tests

[sosthene-nitrokey]

This makes adding future tests easier and adds some test vectors from the gnuk test suite

[sosthene-nitrokey]

So it turns out x25519 key import was incorrect, thanks heiko for noticing it!

x25519 scalar are stored in big endian by gpg, and not in little endian like RFC7718 says.

It also turns out that the gpg tests we had didn't properly test x25519 so we missed it.

[sosthene-nitrokey]

The tests will fail until #89 is fixed