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_hold_persistence_test.go

@@ -51,7 +51,7 @@ func testHoldInvoicePersistence(ht *lntest.HarnessTest) {
 	)
 
 	// Wait for Carol to see the open channel Alice-Bob.
-	ht.AssertTopologyChannelOpen(carol, chanPointBob)
+	ht.AssertChannelInGraph(carol, chanPointBob)
 
 	// Create preimages for all payments we are going to initiate.
 	var preimages []lntypes.Preimage