Drop unnecessary key derivation in route tests

Drop unnecessary key derivation in route tests

In our route tests we need some "random" bytes for the router to
randomize amounts using. We generate this by building an actual
`KeysManager` and then deriving some random bytes using the
`EntropySource` trait. However, `get_route` (what we're normally
testing) doesn't actually use the random bytes, and even if it did,
using a `KeysManager` is just a fancy way of creating a constant,
so there's really no reason to do all the fancy crypto.

Instead, here, we change our routing tests and benchmarks to simply
use `[42; 32]` as the "random" bytes.