itest+lntest: make sure states are cleaned when tests end

This commit changes how the node's state is updated to make sure the
test cleans up the node's state.

Also `testLookupHtlcResolution` is fixed with a cleanup.

itest/lnd_htlc_test.go

@@ -24,6 +24,7 @@ func testLookupHtlcResolution(ht *lntest.HarnessTest) {
 	cp := ht.OpenChannel(
 		alice, carol, lntest.OpenChannelParams{Amt: chanAmt},
 	)
+	defer ht.CloseChannel(alice, cp)
 
 	// Channel should be ready for payments.
 	const payAmt = 100