Add new PKCS12 test vectors

[felixfontein]

Part of #6348; see #6348 (comment).

CC @reaperhulk

[reaperhulk]

One big question here, why use a DSA cert? Nobody uses those and some day we may want to deprecate support, which these vectors will make more of a challenge 😄

[felixfontein]

I basically used the same certs that the other PKCS12 tests were using, and cert2 happens to use a DSA key. Should I replace them by something else (like x509/cryptography.io.pem)?

[reaperhulk]

Yeah please do, sorry about that 😢

[felixfontein]

I'm using the following script to create the vectors: https://gist.github.com/felixfontein/f750763fd5773fbf0ab22af87239aab0 It's based on an earlier version of #6348 which had serialization code.

(Unfortunately it's not idempotent, I guess that is because PKCS12 serialization includes some randomness.)

[felixfontein]

@reaperhulk thanks for reviewing and merging!