itest+lntest: let abandoned channel be either not found or in zombie

index

When abandoning a channel, we remove it from the graph and then add it
to the zombie channel index. However, if we then process a ChannelUpdate
for this channel it will result in us calling `processZombieUpdate`
which will delete the edge from the zombie index. The abandon channel
itest currently asserts that a channel is no longer in the graph by
asserting that when querying for the channel we get a "marked as zombie"
error but to account for the above series of operations, we now also
allow it to just not be found at all ("edge not found").

itest/lnd_misc_test.go

@@ -729,7 +729,7 @@ func testAbandonChannel(ht *lntest.HarnessTest) {
 	require.Len(ht, aliceClosedList.Channels, 1, "alice closed channels")
 
 	// Ensure that the channel can no longer be found in the channel graph.
-	ht.AssertZombieChannel(alice, chanID)
+	ht.AssertNotInGraph(alice, chanID)
 
 	// Make sure the channel is no longer in the channel backup list.
 	err = wait.NoError(func() error {