Integrate tests from wycheproof

[alex]

https://github.com/google/wycheproof:

• aes_cbc_pkcs5_test.json
• aes_cmac_test.json
• aes_gcm_test.json
• chacha20_poly1305_test.json
• dsa_test.json
• ecdh*_test.json
• ecdsa*_test.json
• kw_test.json
• kwp_test.json
• rsa_signature*_test.json
• x25519_test.json

[reaperhulk]

They've published test vectors we can actually use now: https://github.com/google/wycheproof/tree/master/testvectors

[alex]

For anyone interested in working on this, I think we should structure this so CI clones that repo and runs them -- do not copy them all in.

[reaperhulk]

@mithrandi and other distro packagers may have opinions about our tests containing network dependencies?

[alex]

My proposal is that the CI would do the clone, outside the scope of tox/pytest, and then give the location to pytest somehow. Distros could then reproduce this logic.

[mithrandi]

Debian packages are built without network access, so there would need to be a way to disable the downloading. @alex's suggestion seems reasonable since I could ship the vectors in the source package.

[reaperhulk]

When we add the KWP vectors we should test to see if it would catch the 0 mod 8 bug we had.