How to deal with failing test vectors

[mikeboers]

Due to what I believe to be changes to my compiler on OS X, the test vectors for the "kseed" cipher are failing, and the user should be warned that they are incorrect.

I would like to start running every library test on startup, but that will likely be too much overhead (likely 0.1s) for me to be comfortable.

Instead, running each test once, the first time a given cipher/hash/etc. is instantiated (since the descriptor functionality should not be affected) is likely a good tradeoff, raising a RuntimeError if the test does not pass.