lntest+itest: add new method AssertChannelInGraph

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.

itest/lnd_switch_test.go

@@ -415,7 +415,7 @@ func setupScenarioFourNodes(ht *lntest.HarnessTest) *scenarioFourNodes {
 	nodes := []*node.HarnessNode{alice, bob, carol, dave}
 	for _, chanPoint := range resp {
 		for _, node := range nodes {
-			ht.AssertTopologyChannelOpen(node, chanPoint)
+			ht.AssertChannelInGraph(node, chanPoint)
 		}
 	}