Testing other choosy choices

When a `choosy` method has multiple implementations that should work on a given device, it would be nice if there was a documented way to easily *test* all of them.

I say this because I'm attempting to add a test that the adler32 block size isn't too large (by feeding it a file containing 8 KiB of 0xff), but bumping the block size in the default implementation is not doing anything ... which isn't too surprising when I think about it, since the aarch64 processor on my phone definitely has NEON, but is not exactly helpful.