This commit replaces `AssertTopologyChannelOpen` with
`AssertChannelInGraph`, which asserts a given channel edge is found.
`AssertTopologyChannelOpen` only asserts a given edge has been received
via the topology subscription, while we need to make sure the channel is
in the graph before continuing our tests.
When iterating edges, pathfinding checks early whether using an edge
would violate the requested total fee limit for a route. This check is
done on the net amount (an amount the inbound fee is calculated with).
However, a possible next hop's fee discount leads to a reduction in fees
and as such using the net amount leads to assuming a higher cumulative
fee than the route really has, excluding the path erroneously. We
perform the fee limit check on the amount to send, which includes both
inbound and outbound fees. This should be possible as the first hop's
outbound fee is zero and therefore doesn't have to be checked in the
end.
