Switch from miscreant to aes-siv in tests
miscreant is deprecated in favour of aes-siv (according to the author -- this doesn't seem to be publicly documented yet). Additionally, with the version of miscreant we were using it wasn't even possible to resolve the development dependencies, because it depended on sutble 0.3.0, which has been yanked. One thing to note is that aes-siv enforces the nonce length (to 96 bits in our case). Before we were passing in a whole 512-bit key, which AFAICT miscreant happily accepted.