Linux test vectors: assign key length, message size, tweak size randomly

[ciphergoth]

@ebiggers writes:

This PR makes it so that there are two test vectors per message size per key size, so 6 total per message size. Maybe cut that down to 1 per message size per key size, so 3 total per message size? Or even less and just assign key sizes randomly.

not very effective to have 2 test vectors per message length per key length. Problems are more likely to arise for different length inputs, not for different input contents. The xctr test vectors are especially odd, as they only test 32 and 255-byte messages, but have 6 test vectors for each length. We could get better test coverage in the same number (or fewer) of test vectors by focusing on different lengths rather than different contents.