Add find_route test util

Add find_route test util

And use it in the multihop blinded path receive failure test. Will be used in
the next commit to test receiving an invalid blinded final onion payload.

We can't use the existing get_route test util here because blinded payments
rely on the sender adding a random shadow CLTV offset to the final hop; without
this the payment will be failed with cltv-expiry-too-soon.