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.

Author: TheBlueMatt