This test is a sanity check. It verifies that our wrappers over well known
hashes (xxhash, md5, sha256) actually calculate exactly those hashes.
It also checks that the `update()` methods of used hashers are linear with
respect to concatenation: that is, `update(a + b)` must be equivalent to
`update(a); update(b)`. This wasn't relied on before, but now we need to
confirm that hashing fragmented keys without linearizing them won't break
backward compatibility.
