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_res_handoff_test.go

@@ -37,7 +37,7 @@ func testResHandoff(ht *lntest.HarnessTest) {
 	chanPointCarol := ht.OpenChannel(bob, carol, params)
 
 	// Wait for Alice to see the channel edge in the graph.
-	ht.AssertTopologyChannelOpen(alice, chanPointCarol)
+	ht.AssertChannelInGraph(alice, chanPointCarol)
 
 	// We'll create an invoice for Carol that Alice will attempt to pay.
 	// Since Carol is in hodl.commit mode, she won't send back any commit